It's a fairly involved process but its also nearly impossible to unload all the bettas once you have the babies full grown.
You have to introduce the female to the male without allowing them access to each other. A vase is usually used for this. Then wait for the betta to build a bubble nest while fattening the female up. When the female fattens up with eggs then you can introduce them for short periods while they do their business. After they are done remove the female.
It's a very harsh process on female bettas. The tank needs to be bare bottomed with little to no flow.
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor