Get one or more shoulder into the crack and lean your back against the wall. No hands are required, except to move the feet at times. O' 5.6 stuff like that. Now arch your back, bringing your rear end up. Plant the butt and straiten the back, moving up the shoulders.
Hands are somewhat necessary to move the butt, if it's a really tight squeeze.
PRogress on both is slow, but sure, and not difficult. Squueze in, breathe in all of the way, and hold some of it in when you exhale. This may require quite a bit of flexibility to be executed well. It's like a balerena doing hamstring stretches on a bar. Bend the leg all of the way, forceably.