This is not working for me. Something to do with my settings? I'm working in the right layer, I'm in Draw Mode and I've got the drawing tool selected. I can draw the material but the clamping isn't happening. I unlocked the lower layer thinking it needed to be unlocked as well but that didn't work.
around 7:30 (edit). He was drawing a shadow on the toon. You place it where you want the shadow and don't worry about the edge. The clamp will Mask only to the toon.
This got it for me. I didn't know that pre-prep of the lower layer was necessary. This info has filled in the blank for me! And I can copy the screenshot to my notes. ah! Thank you everyone for helping out.