Hi!
Sorry for the late reply - got busy with this Dark Castle run.
I will add the tutorial to my to-do list, but it might be a while before I get around to doing it… in the meantime, the easiest way to learn is probably just jump, then jump again just as you hit the ground - and then sometimes the leg position will be different: that’s when you know you can jump twice again. This is especially useful near portals (like World 1 - I’ve been able to get it with a regular double jump but wasn’t sure if I was using a character that had a jump boost in the skill tree).
The next thing to try is to run against a block/ledge and then press jump - like in one of the World 9 rooms, although there you need to turn around quickly. Again, you need to look at the legs.
And then the last is to jump at a block/ledge and then jump again as you hit it, like before checking the legs to see if it worked.
The hardest is that sometimes these can trigger an extra high jump (needed for the tough one in the poison castle). I haven’t found a way to reliably trigger that one though.
And thanks for the spot on the Gun Dungeon heading 