Here’s the most popular tutorial on how to mod SSF2! This is for 9b, but it works the same way in Beta:
Here’s a more in-depth guide on how to mod Beta:
If you wanna mod the latest version of Beta, here’s a link to the tool shown in the video:
Lastly, here’s a link to a popular modder’s channel! He has a bunch of great tutorials on how to do specific kinds of mods, so if you’re wondering how to do any given thing to your game, he probably has a tutorial for it!