Hard boil an egg, take the yolk out, crush it up, add some water until it's like a paste. Then dip a toothpick in the mixture, then the tank, and the bettas will eat the yolk.
Don't feed too much, it pollutes the tank like crazy. Also, take out any extra they don't eat in a few minutes.
Don't use this as their main diet though, as it probably won't provide enough nutrition.
Instead, I would feed newly hatched BBS. (Baby brine shrimp.)
Good luck! =)