2-3 months before ORD you will be offered the citizenship along
with a time slot to go for ICA interview, you will be given a week
or 2 to decide if you want to take the citizenship.
if you want, sign a letter saying you want.
If you don't want, sign a letter acknowledging that you don't want
and you konw you are only offered ONCE.
But seriously, if you choose to work in Singapore and stay after
your NS, it is not that much harder to get citizenship.
The only difference is that in this offer you will have an
additional letter from SAF stating your good conduct and stuff. And
thuse your chances of gettign citizenship is higher.