To form the crater shape use a spackle (US) or polyfilla (UK), the latter being actually better as it is cellulose-based and does not shrink or warp while drying. Note that my projects that involve a lot of water-based spackling paste are usually constructed on a plywood base to prevent warping
Cover the whole thing with a PVA glue and sprinkle with fine sand:
Unpainted crater
When it is all dry, shake off excessive sand
Spray it all with a black paint
Drybrush it all with a grey paint
Glue some static grass to the base's rim
Blend the line between grass and sand with some black paint
Horses may have many different colours. However, contrary to what many people believe, they are not entirely random. Mane properties are defined by a finite set of pretty well known genes. Let the machine spirit pick them for you! »
How to kitbash the old and the new Citadel industrial "Sector Imperialis" terrain to make the elements look cohesive. And how to make them pipes actually connect afterwards despite the conversions. »