Rough steps for my reference. I will change and refine as I go.

1. Basecoat with Bloodreaver Flesh and Doombull Brown , mixed 4:1.
2. Add more and more Doombull Brown to the mix, gradually shading the recesses. You should eventually end up shading with pure Doombull Brown .
3. Now mix some Rhinox Hide in with the Doombull Brown , shading the deepest recesses only. As before increase the amount of Rhinox Hide as you go deeper and more selective.
4. Thinned Abaddon Black around the eyes and very deep points helps add definition too.
5. With some very thin Word Bearers Red , glaze the areas where blood flow would be close to the surface. On faces this would be on the cheeks, where they would blush and around the mouth.
6. Reestablish the basecoat of 4:1 Bloodreaver Flesh and Doombull Brown on raised/exposed areas, in preparation for highlights.
7. Add a small amount of XV-88 to the mix, highlighting a smaller area. Just add a dab of this paint to the mix, no more than 10%.
8. Now add some Karak Stone to the mix and highlight even more selectively. You can use a little more of this, around 30%.
9. Add final specular highlights of Karak Stone .
10. If any parts of the face are looking a little lifeless, a glaze of Gore-grunta Fur should help add a little warmth.