I'm not a breeder, but here's what I know about breeding, since I seem to be the only one here right now
First off it's a bad idea to breed pet shop bettas or any bad quality bettas. Betta breeders are trying to improve the lines, and breeding bad bettas would just go right against their intentions. Pick high quality stock with a good fin shape, not veiltail. In other words... buy your breeding fish from a real breeder. You'll know then when you see them. Think aquabid or a member of the IBC. Breeders often ship you free females with an order of a male! That's nice isn't it?
You need a large tank, 10g+. 20g would be nice I think. And you need lots of jars for the males and agressive females once they start to nip the other fish. It's rare but they can have up to 500 fry, or even more! An average spawn is probably 100 though.
The breeding tank should be heavily planted, so that the female can hide from the male. Allow the male to become accustomed to the tank for a few days or even maybe a week. Introduce the female in a container so that they cannot contact each other. A cup or a chimney. Let them see each other for a few days. Then you release the female, and if they are interested they should start to mate within a day. This is where the heavily planted tank comes in - once the mating is over and the eggs are in the bubble nest, the male will probably chase the female, seeing her as an intruder. He might kill her. Remove her once you see that they are done and the female is hiding away.
The male will take care of the eggs for a few days and... voila! Fry!
There are mixed opinions on what to feed the fry, they sell formulas in stores, you can make your own cultures, or there's this thing I heard about, which sounds a lot easier - hard-boil an egg, then mix the hard egg yolk with water so that it's a runny paste. Then drop that in there sometimes. It's a bit messy but they like it and it's nutritious.
Very important to keep the water very clean or the fish won't grow up very well. That means looking after it daily and removing any uneaten food and waste, all the time.
Woo.. that was longer than I intended it to be, but there is a lot to know about breeding, and that isn't even a detailed guide - definitely dont' go out and try to breed using the information I told you, it's just a starter, and remember I'm not even a real breeder, this is just stuff I've read.