Here’s a step-by-step that shows first the materials used, then a styrofoam primer, next the main colour: grey textured paint. I usually use acrylics, thinned down with a third water, add some fine sand for texture and a drop of soap to break the surface tension and make the paint creep into every crevice.
The base paint is given a few thin washes, brown and black smudges and a more darkly pigmented layer for the shadow areas. Next, two layers of drybrushing, first light grey, then white.
Finally, all the small and tiny details. For me, the challenge here is knowing when to stop myself before it gets silly.