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
The Games Workshop's Void Shield Generator is a splendid kit but only if you build it differently than recommended on the box. I show step by step how to make a piece of industrial terrain from it, rather than the nouveau riche Christmas tree as recommended officially. »
With a few pieces from a broken hair dryer an a transistor radio I made this Ork Dread to replace the infamous cardboard piece in the 2nd edition of Warhammer 40k »