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
With the Gathering Storm series of books players can enjoy putting power armour, light infantry and sophisticated technology of the Adeptus Mechanicus all in one army. You could have been enjoying the same concoction for 30 years by playing Squats. I will show you how I paint my "Honourable Guild of Plasmaniacs" »
Games Workshop updated their line of texture paint, adding (among other) two products: Astrogranite and Astrogranite Debris that may be useful for simulating imperial architecture. Let's see how they compare to other products I used... »