Re: L Sit training
If you're a gymnast, every day! Hahah.
Depending on where you are is how I would vary it. And how much time you have.
That being said, I'd probably start every other day so 3x/week.
We can use the old 60s method. If you are really weak or bad at them, maybe cut that back to 30 to 45 seconds. Ideally we don't want to work 20 holds of 3 seconds. I would say no more than 10 holds, at least 3-5. Ideally, use a progression that you can hold for 5 seconds because doing 10-20 sets of these will suck and take awhile. That being said, if you have to use 2-3 second holds or maybe just 1 second, use them. In that case, I would probably start out with 30s of time rather than 45 or 60 (I usually scale these up for level of gymnast and ideally a strong compulsory or optional could bang out 2-3 minutes in one set and move on. No idea what I can now because I've neglected it so much [probably 30-60 seconds at least but haven't tried fresh or in awhile so dunno]).
Super basic I do with kids is L-hang and support. I would hope you could do at least those. What's a solid support? Let's say at least 30 if not 60s on PB, dip stands, 2 boxes at 36" (with knees bent back).
Tuck L, Puck (Knees bent but with legs extended ideally thighs still horizontal), L. I'm not a big fan of the single leg L but non gymnasts love them.
Tuck/Pike leg raises in hang or support.
Sit on floor in pike, place hands by thighs, knees or shins and lift heels off floor. Do them as holds or lifts. I like both. 3x10-20 second holds or lifts. I'm not opposed to more but there is often only X amount of time in a gymnastics class/training session.
You're hip flexors are probably going to hate you. Possibly your abs as well.
Try them on one day, rest the next and see how you feel on the third day. Twice a week is fine as a beginner, possibly adding a third day in 3-4 weeks.
You could probably ask Steven Low on the OvercomingGravity reddit sub as well. I think it's in his book (but the next edition is finalizing). My book is downstairs but I'll look later.