Try imagining each folds like malleable cylinders wrapping around the body and then shade them as such. Any surface of the cylinder that is facing the light you, you use light values and any surface in the shadows you just use dark values. If it's too complicated, use references, so that there's less guesswork to do. I think the jaw looks fine so I didn't change anything, and just rendered it. Overall you have a decent base and fun piece to work with, so good luck!