So first you start with a sketch. I did mine in 3B pencil last night while waiting for something to load or something. It's a sperm whale skull! Hence the teeth on the lower jaw.
Then I did a light wash of yellow ochre and burnt umber. I used this freebie sumi brush but it sucked.
Another, slightly darker wash over the darker areas. Switched to a better brush this time.
Then I decided that this baby needed a background, so I mixed some burnt umber with some red and washed away.
Then did the darkest darks on the skull. I didn't have a reference picture, so who knows if the shadowing is accurate or not.
Blend out the darkest darks. Dang, this sketch is wet!
Another layer of darkest darks.
Blend the darks out again.