Molly fish breeding is very simple task. You will need one male and few females with good water conditions. The number of females should be more than the males because males like to chase female, this makes female more stressed. The gestation period of females is of about 30-40 days. Females will start becoming more rounder as the time passes. When the female is about to give birth, her stomach shape will be like \___/ . Then females will give birth to fries. Remember they are live bearers. They will give birth to 30-40 fries at once in time period of 1-2 hrs. Remember to shift the mother or any other fish away from babies otherwise they will eat the babies. You can use breeding nest. Feed crushed flake food to babies and take necessary precautions.
After the birth, separate the mother from any other fish because after birth the molly looses its most of its strength and become easy target to other tank mates. So keep her separated until she gets her strength back. After 1 week of time you can put back her again in main fish tank.