Here's a video tutorial discussing the theory behind how to paint lumps, bumps, divots, holes, pimples, etc. First I
will show what these are supposed to look like using a 3d application to simulate how light reacts to these shapes,
then I'll show you a 2d painting demo using what we learned in the first step, and then a more procedural way to
achieve this look in Photoshop using the Emboss layer style.
Here's a quick cheat sheet: