You should keep them in separate tanks for a little while, feed them flake food and brine shrimp, and then when they are healthy and well fed, you put the tanks next to each other and leave it there for a couple days. Then you move them away from each other so they can't see each other. Then place one betta in the other betta's tank (whichever is the breeding aquarium) but with a tank divider. The male will then be flaring and building a bubble nest. When the bubble nest is pretty big, take out the tank divider and they should be mating in a few days.