How I have done it in the past, is when you think she so close to giving birth, (sluggish, slow swimming patterns, vertical swimming, and her spot beneath her anal fin is large) I cup her, (I had a mamma molly abort because I used a net once...) instead use the net to guide her into the cup filled with water.
If your tank is big enough, I would use established tank water in the smaller container/tank. The fry will need a heater, an Airstone and a small filter, preferably a sponge filter so they don't get sucked up in it.
After mamma is done, ( she will start swimming around like crazy hungry swimming, return her to he main tank, or she will eat the babies.
Another option is buying a breeder box that floats in your main tank so the big fish won't eat them, so you don't have to buy a separate set up.
Often times this is very stressful on the momma and the babies. So I recommend a separate smaller tank/container.
Any other questions?
Raising fry is hard work. But it's really rewarding.