I did it once, years ago, but I did not do it, um...responsibly, exactly. What I mean by that is I bred two unremarkable pet store bettas, i.e. not genetic lines that the world really needed any more of. I was keeping two or three females in a community tank at the time, and I set up a male in a warm, partially filled 10g w/ a 1/2 plastic cup floating (but anchored to the side with tape) for the bubble nest. I used a glass chimney for a hurricane lamp to introduce the female. Things progressed as the books said they should. After merciless culling, several new females for the community tank, and efforts to give away males to anybody I knew, as many as they would take, I ended up with (I think) 9 new (unremarkable, to anyone but me) male bettas of my own to care for, and of course a sense of accomplishment....
Before anyone flames me, yes I have learned that breeding low-quality bettas is not a good idea, and I've also learned the related lesson that it is cruel and irresponsible to breed fish when you cannot keep, sell, or give away enough of them to avoid "culling" healthy juveniles. Like I said, years ago. Once.
But beyond the ethical problems? It was extremely interesting, and a lot of fun.