Here’s one I started of Immortan Joe from Mad Max: Fury Road.
Based on this image:
Basically what I did is put the image on the bottom layer of photoshop, then draw on top of it using a pen tablet and four-ish shades of gray.
Then once I have something that looks right, I remove the bottom layer and we end up with grays, like this.
This could use a bunch more polish and some color.
Not sure I will do it, but who knows?