Something I wrote elsewhere:

Anapanasati Sutta

My comments: Anapanasati, or mindfulness of breathing, is the most well-known and commonly practiced Buddhist meditation method. Some teachers have taught anapanasati as some 'basic' meditation as if it is only meant to be practiced by very beginning students, or as if it is a kindergarten method that cannot lead you anywhere far. Many people thus have the misconception that they should give up this practice for "higher" or "more profound" practices as soon as possible. To me this is not necessarily the case, I have seen (and many find) that anapanasati continues to be a very powerful practice for beginners and "advanced meditators" (though I am nowhere near being an "advanced meditator" yet) alike. At the same time, I am not at all suggesting that Buddhist meditation is confined to "anapanasati" - even in the Pali suttas, dozens of meditation subjects were prescribed by the Buddha depending on the inclinations and needs of different people, needless to speak about Mahayana and Vajrayana methods. "Anapanasati" however is prescribed as an antidote for overcoming discursive thinking, which I think is a pretty universal problem. This method is conducive for the development of tranquillity and insight.

However I should also add: I recommend switching to mindfulness of the bare sensations of breathing instead of 'counting the breath' as soon as possible, because counting is still conceptual and should be dropped for deeper development of tranquility and insight whenever possible. Notice that the Buddha did not teach "counting the breath" but instead he taught "mindfulness of breathing".

It should be noted that the Buddha himself practiced anapanasati as his own meditation subject - it was by practicing anapanasati that he attained the three knowledges (knowledge of his past lives, knowledge of sentient beings' karma and rebirth destinations, and the knowledge of the eradication of defilements, i.e. liberation). And it was anapanasati that he practiced even after his enlightenment (his continuous practice of such meditations are for two reasons which he stated: cultivating anapanasati results in a pleasant abiding, and setting up a good example for his students). So from the day he practiced it, he had never abandoned this practice. Obviously he did not see this method as "lowly/base" but as a potent method that when practiced can result in the attainment of Nirvana.

For those who prefer a more "choiceless" form of awareness practice, that is, being choicelessly aware of everything that comes to perception, they may wonder "why should I bother practicing mindfulness of breathing? Isn't this some contrived meditation that is not so helpful?" My answer is that even if you have non-dual insight, even if spacious, pervasive, non-dual awareness is experienced quite effortlessly, mindfulness of breathing continues to be very beneficial. You may have clear realization of your own mind essence and nature, you may be a direct path practitioner (who may have a tendency to look down upon 'gradual methods'), but still you may find benefits in revisiting basic meditations like anapanasati.

For example, Thusness told me anapanasati is even more important for me now than certain vipassana methods like Goenka or body scan (which is useful for bringing presence into the 'foreground' but this is already quite natural for me), because anapanasati is conducive to the development of relaxation and concentration. It also leads to a deepening into the actualization/experience of no-self. Part of his rather recent advice to me was, "...you must spend some time, not to have too much thoughts. Not to have too much discursive thoughts… lessen the thoughts. Then feel this raw sensation. This is total presence. Understand? You must give yourself some quality hours to keep on experiencing this. So when you practice like sensing your breath. Let’s say, (breathes) just this total sensation, then release. Then breathe… you just need to focus, that’s enough. Then feel the sensation. Focus. Then you incorporate your view into it. Step by step. Then you will be concentrated because you do not have other discursive thoughts. Then you will have the total experience of just this breath. Keep on practicing, you know? Make this into a kind of homework."

A little personal history: I discovered this teaching by Buddha - the Anapanasati sutta, only quite some time after I started doing this meditation (I still do, today). And when I first read it, I was very happy because the 16 steps that he described was just so precise and resonates with my experience. But I must say with regret that I am not a very skilled or hardworking (it does take discipline for a consistent meditation regiment) meditator like many, though I am hoping to spend more time in meditating (rather than writing long essays like this, lol). As for when I started this practice, it was when I was 15 years old. The second time I ever did anapanasati (the first time was when I was being taught this meditation by my local dharma teacher), I entered into a state of incredible bliss pervading the entire body and a sense of bodily dissolution. At that time I had little idea what exactly I was experiencing. Later Thusness informed me that what I experienced was the bliss of jhana (he was rather surprised I could experience this since it usually does not happen so quickly). In any case, that certainly strengthened my faith and confidence in such practices, and I hope you too will find much benefits in Anapanasati. In fact, I am sure you will, if you keep a consistent and regular practise.

Enjoy reading/breathing.

...............

MN 118
PTS: M iii 78
Anapanasati Sutta: Mindfulness of Breathing
translated from the Pali by
Thanissaro Bhikkhu
© 2006–2012

I have heard that on one occasion the Blessed One was staying at Savatthi in the Eastern Monastery, the palace of Migara's mother, together with many well-known elder disciples — with Ven. Sariputta, Ven. Maha Moggallana, Ven. Maha Kassapa, Ven. Maha Kaccana, Ven. Maha Kotthita, Ven. Maha Kappina, Ven. Maha Cunda, Ven. Revata, Ven. Ananda, and other well-known elder disciples. On that occasion the elder monks were teaching & instructing. Some elder monks were teaching & instructing ten monks, some were teaching & instructing twenty monks, some were teaching & instructing thirty monks, some were teaching & instructing forty monks. The new monks, being taught & instructed by the elder monks, were discerning grand, successive distinctions.

Now on that occasion — the Uposatha day of the fifteenth, the full-moon night of the Pavarana ceremony — the Blessed One was seated in the open air surrounded by the community of monks. Surveying the silent community of monks, he addressed them:

"Monks, I am content with this practice. I am content at heart with this practice. So arouse even more intense persistence for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. I will remain right here at Savatthi [for another month] through the 'White Water-lily' Month, the fourth month of the rains."

The monks in the countryside heard, "The Blessed One, they say, will remain right there at Savatthi through the White Water-lily Month, the fourth month of the rains." So they left for Savatthi to see the Blessed One.

Then the elder monks taught & instructed the new monks even more intensely. Some elder monks were teaching & instructing ten monks, some were teaching & instructing twenty monks, some were teaching & instructing thirty monks, some were teaching & instructing forty monks. The new monks, being taught & instructed by the elder monks, were discerning grand, successive distinctions.

Now on that occasion — the Uposatha day of the fifteenth, the full-moon night of the White Water-lily Month, the fourth month of the rains — the Blessed One was seated in the open air surrounded by the community of monks. Surveying the silent community of monks, he addressed them:

"Monks, this assembly is free from idle chatter, devoid of idle chatter, and is established on pure heartwood: such is this community of monks, such is this assembly. The sort of assembly that is worthy of gifts, worthy of hospitality, worthy of offerings, worthy of respect, an incomparable field of merit for the world: such is this community of monks, such is this assembly. The sort of assembly to which a small gift, when given, becomes great, and a great gift greater: such is this community of monks, such is this assembly. The sort of assembly that it is rare to see in the world: such is this community of monks, such is this assembly — the sort of assembly that it would be worth traveling for leagues, taking along provisions, in order to see.

"In this community of monks there are monks who are arahants, whose mental effluents are ended, who have reached fulfillment, done the task, laid down the burden, attained the true goal, laid to waste the fetter of becoming, and who are released through right gnosis: such are the monks in this community of monks.

"In this community of monks there are monks who, with the wasting away of the five lower fetters, are due to be reborn [in the Pure Abodes], there to be totally unbound, destined never again to return from that world: such are the monks in this community of monks.

"In this community of monks there are monks who, with the wasting away of [the first] three fetters, and with the attenuation of passion, aversion, & delusion, are once-returners, who — on returning only once more to this world — will make an ending to stress: such are the monks in this community of monks.

"In this community of monks there are monks who, with the wasting away of [the first] three fetters, are stream-winners, steadfast, never again destined for states of woe, headed for self-awakening: such are the monks in this community of monks.

"In this community of monks there are monks who remain devoted to the development of the four frames of reference... the four right exertions... the four bases of power... the five faculties... the five strengths... the seven factors for awakening... the noble eightfold path: such are the monks in this community of monks.

"In this community of monks there are monks who remain devoted to the development of good will... compassion... appreciation... equanimity... [the perception of the] foulness [of the body]... the perception of inconstancy: such are the monks in this community of monks.

"In this community of monks there are monks who remain devoted to mindfulness of in-&-out breathing.

"Mindfulness of in-&-out breathing, when developed & pursued, is of great fruit, of great benefit. Mindfulness of in-&-out breathing, when developed & pursued, brings the four frames of reference to their culmination. The four frames of reference, when developed & pursued, bring the seven factors for awakening to their culmination. The seven factors for awakening, when developed & pursued, bring clear knowing & release to their culmination.
Mindfulness of In-&-Out Breathing

"Now how is mindfulness of in-&-out breathing developed & pursued so as to be of great fruit, of great benefit?

"There is the case where a monk, having gone to the wilderness, to the shade of a tree, or to an empty building, sits down folding his legs crosswise, holding his body erect, and setting mindfulness to the fore.[1] Always mindful, he breathes in; mindful he breathes out.

"[1] Breathing in long, he discerns, 'I am breathing in long'; or breathing out long, he discerns, 'I am breathing out long.' [2] Or breathing in short, he discerns, 'I am breathing in short'; or breathing out short, he discerns, 'I am breathing out short.' [3] He trains himself, 'I will breathe in sensitive to the entire body.'[2] He trains himself, 'I will breathe out sensitive to the entire body.' [4] He trains himself, 'I will breathe in calming bodily fabrication.'[3] He trains himself, 'I will breathe out calming bodily fabrication.'

"[5] He trains himself, 'I will breathe in sensitive to rapture.' He trains himself, 'I will breathe out sensitive to rapture.' [6] He trains himself, 'I will breathe in sensitive to pleasure.' He trains himself, 'I will breathe out sensitive to pleasure.' [7] He trains himself, 'I will breathe in sensitive to mental fabrication.'[4] He trains himself, 'I will breathe out sensitive to mental fabrication.' [8] He trains himself, 'I will breathe in calming mental fabrication.' He trains himself, 'I will breathe out calming mental fabrication.'

"[9] He trains himself, 'I will breathe in sensitive to the mind.' He trains himself, 'I will breathe out sensitive to the mind.' [10] He trains himself, 'I will breathe in satisfying the mind.' He trains himself, 'I will breathe out satisfying the mind.' [11] He trains himself, 'I will breathe in steadying the mind.' He trains himself, 'I will breathe out steadying the mind.' [12] He trains himself, 'I will breathe in releasing the mind.' He trains himself, 'I will breathe out releasing the mind.'[5]

"[13] He trains himself, 'I will breathe in focusing on inconstancy.' He trains himself, 'I will breathe out focusing on inconstancy.' [14] He trains himself, 'I will breathe in focusing on dispassion [literally, fading].' He trains himself, 'I will breathe out focusing on dispassion.' [15] He trains himself, 'I will breathe in focusing on cessation.' He trains himself, 'I will breathe out focusing on cessation.' [16] He trains himself, 'I will breathe in focusing on relinquishment.' He trains himself, 'I will breathe out focusing on relinquishment.'

"This is how mindfulness of in-&-out breathing is developed & pursued so as to be of great fruit, of great benefit.
The Four Frames of Reference

"And how is mindfulness of in-&-out breathing developed & pursued so as to bring the four frames of reference to their culmination?

"[1] On whatever occasion a monk breathing in long discerns, 'I am breathing in long'; or breathing out long, discerns, 'I am breathing out long'; or breathing in short, discerns, 'I am breathing in short'; or breathing out short, discerns, 'I am breathing out short'; trains himself, 'I will breathe in...&... out sensitive to the entire body'; trains himself, 'I will breathe in...&...out calming bodily fabrication': On that occasion the monk remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world. I tell you, monks, that this — the in-&-out breath — is classed as a body among bodies, which is why the monk on that occasion remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world.

"[2] On whatever occasion a monk trains himself, 'I will breathe in...&...out sensitive to rapture'; trains himself, 'I will breathe in...&...out sensitive to pleasure'; trains himself, 'I will breathe in...&...out sensitive to mental fabrication'; trains himself, 'I will breathe in...&...out calming mental fabrication': On that occasion the monk remains focused on feelings in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world. I tell you, monks, that this — careful attention to in-&-out breaths — is classed as a feeling among feelings,[6] which is why the monk on that occasion remains focused on feelings in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world.

"[3] On whatever occasion a monk trains himself, 'I will breathe in...&...out sensitive to the mind'; trains himself, 'I will breathe in...&...out satisfying the mind'; trains himself, 'I will breathe in...&...out steadying the mind'; trains himself, 'I will breathe in...&...out releasing the mind': On that occasion the monk remains focused on the mind in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world. I don't say that there is mindfulness of in-&-out breathing in one of lapsed mindfulness and no alertness, which is why the monk on that occasion remains focused on the mind in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world.

"[4] On whatever occasion a monk trains himself, 'I will breathe in...&...out focusing on inconstancy'; trains himself, 'I will breathe in...&...out focusing on dispassion'; trains himself, 'I will breathe in...&...out focusing on cessation'; trains himself, 'I will breathe in...&...out focusing on relinquishment': On that occasion the monk remains focused on mental qualities in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world. He who sees with discernment the abandoning of greed & distress is one who watches carefully with equanimity, which is why the monk on that occasion remains focused on mental qualities in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world.

"This is how mindfulness of in-&-out breathing is developed & pursued so as to bring the four frames of reference to their culmination.
The Seven Factors for Awakening

"And how are the four frames of reference developed & pursued so as to bring the seven factors for awakening to their culmination?

"[1] On whatever occasion the monk remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world, on that occasion his mindfulness is steady & without lapse. When his mindfulness is steady & without lapse, then mindfulness as a factor for awakening becomes aroused. He develops it, and for him it goes to the culmination of its development.

"[2] Remaining mindful in this way, he examines, analyzes, & comes to a comprehension of that quality with discernment. When he remains mindful in this way, examining, analyzing, & coming to a comprehension of that quality with discernment, then analysis of qualities as a factor for awakening becomes aroused. He develops it, and for him it goes to the culmination of its development.

"[3] In one who examines, analyzes, & comes to a comprehension of that quality with discernment, persistence is aroused unflaggingly. When persistence is aroused unflaggingly in one who examines, analyzes, & comes to a comprehension of that quality with discernment, then persistence as a factor for awakening becomes aroused. He develops it, and for him it goes to the culmination of its development.

"[4] In one whose persistence is aroused, a rapture not-of-the-flesh arises. When a rapture not-of-the-flesh arises in one whose persistence is aroused, then rapture as a factor for awakening becomes aroused. He develops it, and for him it goes to the culmination of its development.

"[5] For one enraptured at heart, the body grows calm and the mind grows calm. When the body & mind of a monk enraptured at heart grow calm, then serenity as a factor for awakening becomes aroused. He develops it, and for him it goes to the culmination of its development.

"[6] For one who is at ease — his body calmed — the mind becomes concentrated. When the mind of one who is at ease — his body calmed — becomes concentrated, then concentration as a factor for awakening becomes aroused. He develops it, and for him it goes to the culmination of its development.

"[7] He carefully watches the mind thus concentrated with equanimity. When he carefully watches the mind thus concentrated with equanimity, equanimity as a factor for awakening becomes aroused. He develops it, and for him it goes to the culmination of its development.

(Similarly with the other three frames of reference: feelings, mind, & mental qualities.)

"This is how the four frames of reference are developed & pursued so as to bring the seven factors for awakening to their culmination.
Clear Knowing & Release

"And how are the seven factors for awakening developed & pursued so as to bring clear knowing & release to their culmination? There is the case where a monk develops mindfulness as a factor for awakening dependent on seclusion, dependent on dispassion, dependent on cessation, resulting in relinquishment. He develops analysis of qualities as a factor for awakening... persistence as a factor for awakening... rapture as a factor for awakening... serenity as a factor for awakening... concentration as a factor for awakening... equanimity as a factor for awakening dependent on seclusion, dependent on dispassion, dependent on cessation, resulting in relinquishment.

"This is how the seven factors for awakening are developed & pursued so as to bring clear knowing & release to their culmination."

That is what the Blessed One said. Gratified, the monks delighted in the Blessed One's words.
Notes

1.
To the fore (parimukham): The Abhidhamma takes an etymological approach to this term, defining it as around (pari-) the mouth (mukham). In the Vinaya, however, it is used in a context (Cv.V.27.4) where it undoubtedly means the front of the chest. There is also the possibility that the term could be used idiomatically as "to the front," which is how I have translated it here.
2.
The commentaries insist that "body" here means the breath, but this is unlikely in this context, for the next step — without further explanation — refers to the breath as "bodily fabrication." If the Buddha were using two different terms to refer to the breath in such close proximity, he would have been careful to signal that he was redefining his terms (as he does below, when explaining that the first four steps in breath meditation correspond to the practice of focusing on the body in and of itself as a frame of reference). The step of breathing in and out sensitive to the entire body relates to the many similes in the suttas depicting jhana as a state of whole-body awareness (see MN 119).
3.
"In-&-out breaths are bodily; these are things tied up with the body. That's why in-&-out breaths are bodily fabrications." — MN 44.
4.
"Perceptions & feelings are mental; these are things tied up with the mind. That's why perceptions & feelings are mental fabrications." — MN 44.
5.
AN 9.34 shows how the mind, step by step, is temporarily released from burdensome mental states of greater and greater refinement as it advances through the stages of jhana.
6.
As this shows, a meditator focusing on feelings in themselves as a frame of reference should not abandon the breath as the basis for his/her concentration.

See also: SN 54.8.