Use Ushabti Bone , using several thin coats as needed.
Begin with an all over shade of Agrax Earthshade mixed with Contrast Medium at about 1:3 parts.
Repeat the previous layer, but work only into the recesses
Using Snakebite Leather mixed with Contrast Medium , build up darker areas in the deepest recesses and to the sides of raised points
Use Ushabti Bone to establish edges. Keep the paint thin to allow the paint to blend
Thinner highlights with Screaming Skull
Final spot highlights with Screaming Skull with White Scar mixed (about 1:1 but you can add more or less depending on where on the model)
One very final glaze over all of the bone, using Contrast Medium with a tiny spot of Snakebite Leather in (as if the Contrast Medium is simply contaminated). This provides and even finish and ties the colours together.
Highlight the uppermost points with tiny dots of White Scar , which you can add some Screaming Skull to if you wish to be less stark. Focus only on highest points.