You have no business attempting to breed Bettas! First, learn something about them! For the love of fish, at least read a book about the species! You can find one at any book store, pet store, or public library.
To answer your question in a nutshell, when Bettas mate, the male squeezes the eggs out of her, fertilizes them, and puts them in the bubble nest. He watches over them until the eggs hatch and the fry are free-swimming.
This shouldn't be attempted until both fish have been properly conditioned and are ready for mating. Then they must be properly introduced at the right time. Only an experienced breeder will know how to do this. An amateur runs the risk of one of his fish being killed!
If, by some stroke of "luck" a successful mating did occur, what do you plan to do with all those fry? The males need to be separated and put into separate tanks. Do you have the room and money for the dozens of tanks you are going to need? Do you think by placing an ad, the world is going to beat a path to your door? Why would they? Most people wanting to buy Bettas will just go down to Petsmart and get one. You are going to wind up with potentially hundreds of fish you can't get rid of!
You will never make anything even close to the amount of money it will cost you to raise the fry, if you do get your fish to mate. Give it up.
