I am not responsible for dumbassery or screwups:
1) Remove head, hands, boots, leather gauntlets. Set them aside. Yes, this is a full undress of the figure.
2) Remove belt. There is a small snap in the back, do that one first. The tiny snap probably could fit through the buckle but you don't really need to do that. Unsnap the front two snaps and there's enough slack to slip the belt down over the legs.
3) Trousers off. The action figure, not yours.
4) Cap's shirt is attached to the web "underwear" in ~ 5 places by a simple stitch. I chose to cut the web underwear instead of risking stitches on the shirt.
5) There is a criss-cross buckle on Cap's back and tiny snaps. Undo them. This will give you a bit of slack for getting the shirt of. It's all one piece, don't try to disassemble it.
6) Carefully work the arms and shoulders so the arms can go straight up over his head. Patience.
7) Wiggle the shirt off. Again, not yours.
8) Remove the big-ass black foam pads from inside the upper arms.
Now put it back together.