Well, since Decroded is no computer, you are stuck with me running on little sleep
Admittedly I did rush through this a bit and skipped a few steps in Decroded's how-to guide to asteroids (mainly step 7). Hopefully this still gets the idea across.
EDIT: Now that I look at it, the craters look like herpes or smallpox. Widen out the cast shadow and go easier on the highlight and the dark shade inside the crater than I did to avoid that.