I only have the Citadel paints so I don't know what your equivalents would be but here's what I did:
1. Prime Black
2. Undercoat Zandri Dust.
3. Layer 50:50 Zandri and Screaming Skull
4. Wash with Agrax Earthenshade
5. Highlight Screaming Skull.
6. Highlight 50:50 Screaming Skull / White Scar
7. Highlight White Scar.
Here's how my biker and sarge turned out. Please note that I left too much of the undercoat showing in the deepest recesses. I need to go back up and touch these up to make the folds less sharp.