get a breeder and stick the females who look ready to pop in there. remember to get one with seperations so that you can add more than one female and have them not eat the babies. try and get a Lee's 3-Way Guppy Breeder.
Bettas are rather easy, but it's a dangerous process for the male and female. you'd need seperate tanks for each one, a ten gallon breeding tank filled halfway with water, a styrofoam cup for the male to build a nest under, bushy silk plants in one corner, a breeder trap, and of course Bettas!
take the male and add him to your breeding tank, let him get a feel for it. add the female in the breeder trap and let them flare and show off and wait for hte male to start building a nest under his cup. once he starts release the female. there'll be chasing and nipping, but the girl will hide in the plants and the boy will make his nest and show off. once the nest is big and if the female submits (vertical bars, swimming head-down) the two will embrace under the nest and the male will squeeze the bejeebers outta the female. she'll look stunned while he collects the eggs and sticks them in the nest. this could go on for ages, but when it's done you MUST remove the female and add her to a cycling hospital tank to tend to her exhaustion and wounds.
=) it's a fun hobby, but remember the fry need care too! some large tanks are needed for the female fry, or really big sterlite bins outfitted with sponge filters. males need gallon tanks, jars, bowls, seperate breeder traps in other communities, or a divided tank. they can be told apart at 2 months, and sold at 4 i believe.