Just wrapped up a severed head shoulder bag and figured I'd share what I learned, since this kind of build looks intimidating but is honestly mostly patience.
The trick to the wrinkled, shrunken-skin look is casing your top layer relly well, then pushing the wrinkles in by hand while it's wet and letting it dry slowly. I laid a thinner skin over a veg-tan base so the structure holds its shape while the face gets all the texture. Don't try to make the stitching perfect either. Wonky, visible seams acutally sell the creepy look better than clean ones whould. Leave the bottom edges raw and cut them jagged so they hang like strips. For the color, work in thin layers of leather dye first to build up the old, dirty tone, then hit the high points like the brow and nose with lighter paint after. Doing it in reverse looks flat. One more thing, sketch the face on the leather with a ballpoint before you commit to anything. My first attempt at the eyes ended up lopsided and I'm glad I hadn't cut yet.