Mollies breed easily. Ideally, keep them in slightly brackish water, and turn the heater up a notch or two. Mollies like warmer water than most tropicals. 28-30 C will do nicely. Throw in some floating plants. Hornwort is ideal, but Canadian pondweed will do nicely. Basically something to provide the baby fish with cover. Put the females in the tank, let them settle in, and feed them up for a few days. Then add the male. He'll take care of business (assuming these are virgin females).
After a week or so, remove the male. You don't want him harassing the female, and removing him will reduce the stress on the females, reducing the chances of miscarriages. Gestation is about a month, +/- depending on the temperature and quality of the food.
The "gravid spot" is not nearly as easy to see on all fishes as you might imagine. Instead, look out for obviously larger girth around the belly. I find that a few days before livebearers drop their young they become a little shyer than normal, and hide among the plants.
Never, EVER put a molly in a breeding trap. She will hate you forever. That's what the plants are for; the babies will swim into the plants and they can easily be fished out and removed to another tank, if that's what you want to do.
Once the babies are out, with mollies, keeping them warm and making regular water changes in the key to getting big fish. Mollies are VERY easily stunted. With sailfin mollies, the size of the dorsal fin is said to be proportional to the aquarium they are raised in, with overcrowded fish never becoming as nice as ones given more room.
Cheers,
Neale
