I forgot to upload all of these! Turns out I have been drawing, you guys!
Main points: Smudge tool solves everything. Think of your "paint" like clay. If this drawing were a sculpture, which way would the planes be facing?
Form rendering is really about consistency. Under strong direct light, all planes facing same direction should have same value.
Process: Block out with opaque values. Smudge over transitions. Go in with tiny brush and crosshatch like you would with a pencil.
Last one inspired by Pan's work. I learned a lot from seeing his studies. Mostly that you have to put in the time with a tiny brush if you want the form to be precise. There are no shortcuts.