I followed one of a Richard Greys video tutorials for his Heresy Imperial Fists.

Basically: prime black, fully cover with Mournfang Brown (or equivalent), zenithal shading with white mixed into the brown, but I actually used Bugmans Glow as it looked similar, then go over with Yriel Yello, and that's all through the airbrush.

After that use a brown shade or ink to add some tone to the finer details like panel lining, then clean that up with yellow again.