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
I look at the recently released set of "Citadel Skulls" and use it to convert my Chaos Rhino so that it is full of skulls! Some Chaos Squats manage to photobomb the video. »
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... »