If you can, L-sits FTW. If you can't do them on the ground, you can use dumbbells, build parallettes out of PVC, or use a dip-machine while you progress. I'm on the club gymnastics team and those are brutal. You can search around on gymnasticbodies.com and you'll find a bunch of useful stuff on progressions - I can tell you the progression if you're interested, but I'd definitely recommend it. Really humbling haha.