1. Texture itself. Stirland Mud here, but any similar paste will do. I thin it a bit with water on a base and spread with a diy tool, which is just a ground old x-acto blade glued into an old brush handle. You can sprinkle some coarse send on top for varied texture. Wipe the base edge clean before it dries.
2. Black primer, then German Grey. I'd attach the model after this stage and work up the next step onto the lower legs & cloaks, etc.
3. Pigment "wash". Burnt Umber, Old Rust, Natural Sienna. Thin BU with water & airbrush thinner mix to consistency resembling a wash and spread it over entire base, then sprinkle all 3 pigments sparingly on top and work them in. Let dry completely.
4. Light drybrush. Medium Sea Grey.