I recently made some rocky hills, using the same technique that I used to (re)base my Space Wolves army on 32mm bases. The techniques is simple. I’m using roughcast to give some volume and smooth the surface, then while it’s still drying I include some small gravels and slate rocks. Once dried, I brush some of my liquid adhesive on the flat surface and poor a generous amount of sand on it. I leave it dry without removing the excess of sand. Thanks to capillarity, the adhesive will impregnate the sand on a thicker level. This is one of the secret to have a really nice sand effet and not a flat one. So, one day latter, I come back and I remove the excess of sand then I carry on painting the whole hill or base with a brown primer. The slate rocks were airbrushed with a strong grey, then highlighted with a bluish pale grey. The final step consists in dry brushing with beige the whole surface (sand gravel, slate rocks). Depending what type of terrain you are making you can then add some static grass or some pigments (ocher, violet, etc).