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:
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
Do you know Humbrol 130 and Vallejo Metal Medium? They reflect light differently to standard paints and offer new possibilities. Like replicating the old and missed by many Boltgun Metal effect. »
I had a great fun installing custom LED lighting and doing a lot of scratch-building and converting, that went into the vehicles, their crews and others. These Ironhead Squat Prospectors were my entry in my local Games Workshop store. »