He'd never done terrain before (or much drybrushing) so I told him what to do!
Craters were all sprayed chocolate(ish) brown (a colour wolflord havoc was spraying all his skaven and I..er...borrowed it...).
All the following is done wth a large 1/2" brush
1st drybrush all over = tallern flesh
2nd dry brush not to the edges and lighter = tausept ochre
Final drybrush around rim and any prominant edges = dheneb stone
In the middle of the crater heavily drybrush a circle using black but getting lighter as the circle expands (effectively blending the stain in). Drybrush streaks out from the middle to the centre with black too (lightly... getting very light to the eges, again blending the stains to the brown background).
Edges flocked with static grass/coral sand mix (same as I use on my marines). Patches of grass on the rocks are a
really nice flock from TSS