The first thing you can do is make sure youre strong enough.. a good measure is if you can do straddle throughs on the bar - if you dont know what those are.. imagine a V-Up on the bar, straddled, where you pull your legs past your arms (into a stalder position), and back.
Having the stamina to complete 2-3 sets of 5-7 are good, 2-4 sets of 10 are probably your target.
If your hip flexors are weak, doing cris-cross hangs on the bar might be a good way to work those (basically, hang in a straddle, cross your legs back and forth alternative legs to go on top).
After you are strong enough (and even before if youre feeling frisky - but if you start PLEASE continue to do the conditioning to prevent injury) you can get your hands on a strap bar.
Start by doing straddle circle swings, eventually doing straddle circles around the bar.
For the actual stalder/endo, youre gonna have to start opening up, and not circling the whole bar... the best way to learn is to move through the motion the best you can, and feel more confident on the open on both sides as you progress.. It's like being on a swing, use the pump and pull from each side (in this case, the open and close from each side) to build up the confidence and the muscles and control to open for the stalder/endo.
If you are moderately strong and capable, and you practice every other day or so, you can expect to pick them up in as little as a month (looking ugly), and have some good ones that you can endo drop stalder drop endo (yes, that's 3 in a row, changing directions) in about 3-5 months. When you are there, before going to the bar without the straps, do 3 stalders or 3 endos in a row.
From there, put them on the bar with a spot (and a soft mat under you), and you should have them pretty easily.
This is a patience skill, and a muscle memory skill. It will take time, and will take understanding that you may need to work on it for a while, then come back to it. However, if you are truly dedicated to competing them, and completing them, this is the best method I know. It also teaches you both endos and stalders! Two for one special!!
However, I find it the most safe, as it allows you to safely progress without pushing yourself past where you should be.
The general concept of the stalder and endo - from a handstand, planche to about 45* (about 2 o'clock), then deep straddle as you pull down on the bar (compression stage) (6 o'clock). From there, you should begin decompressing at about 7-8 o'clock. As you open up, the swing should carry you over to the handstand.
Endos work the same backwards, though it does require a lot more shoulder control, since you have to wait until your body reaches about 15-20* (1/2 way between 10 and 11 o'clock-ish) before you begin to compress and close the shoulder angle. However, the open is similarly as early.
Good luck to you!!
Keep us updated on your progress. They are such a neat skill!!
Thanks for all of this. I absolutely love stalders, but haven't taught them before. I've got a promising group of younger optionals that I'd love to start training for these, so I appreicate all of the drills. I particularly like the strength ones. And we do have a strap bar, so that will be really helpful.
A common frustration during the skill is to think about opening the toes towards the ceiling.
This is a totally un-natural movement. The goal during the unfold is to get the toes as far away from the body as possible. The tight compression of the stalder speeds up the circle around the bar so much, that it actually has to be slowed by opening away from the bar! During the decompression phase, starting at around 7 o'clock, the body should fully extend at around 10 o'clock (or so...) The rest of the swing will naturally carry over to a tall handstand.