Yours truly did a DIY trip to France, but not to Lourdes, but
somewhere else lah. But I can offer a few hints.
Get yourself from Singapore to Paris. There are a few ways of doing
it. One is of course to fly straight to Paris. Another way is you
fly to London and zip across with the Eurostar. Or you can go to
Brussels or Amsterdam too and then catch a train to Paris.
For the flight to Paris, check out Malaysia airlines or Thai
Airways. These two are the cheapest.
From Paris you take a train to Lourdes. The mainline train station
to take the train to Lourdes is Montparnasse.
You can either show up on the day of travel itself and buy tickets
from the counter, or you can book in advance at the SNCF website.
Booking in advance is ALWAYS cheaper. Read this website for
details:
http://www.tripadvisor.com/Travel-g187147-c3019/Paris:France:Booking.On.Sncf.html
It is always useful to learn a few french phrases. You should also
expect to spend about 2.5k to 3k for this trip.