An L seat is one of the best exercises to build your abdominal muscles and core. It's important to get comfortable with this before moving on to some of advanced static moves. I find the dip bars are the best place to learn these; they give you more space than parallettes and the beginning position is easier to hold than hanging from a pull up bar.
01 Tucked L seat
From the top of a dip position, tuck your knees in to your chest, but only so far that your thighs are parallel to the ground.
02 Half L seat
Get in to a tucked L seat position. From here, keep one leg tucked and straighten the other one. Your straight leg should be parallel to the ground (if this is too hard, bend your lower leg to a 45 degree angle).
03 Advanced tucked L seat
Again, start in a tucked L seat position. Start to extend both legs; keep your thighs parallel to the ground, but bend your lower legs to a 45 degree angle.
04 Tuck to L seat
Similar to above, get in to a tucked L seat and extend your legs. Instead of stopping at 45 degrees, straighten your legs the whole way so they're parallel to the ground.
05 L seat
Once you can do all of the above, all that's left in getting in to an L seat without tucking first. From the top of a dip position, keep both legs straight, raise them 90 degrees and then hold.