My mid-tier effort in realistic skin movement, 32 stages in the file both in the original half model work-in-progress version, and how i later mirrored everything and made a couple asymmetric little changes, especially at the end of the sequence with the skin flapping over. This newer sequence negates the need for a flaccid and an erect series of states in favor of a more complete / fluid movement, counting on you having the right rig to deform this into the final shape. I recommend a lattice deformer / modifier, maybe conforming to curves.