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
Thousands of 40k aficionados gathered their jaws from the floor, as Forgeworld released a new Squat model, after 20-something years of GW failing even to acknowledge, that the race ever existed. Grendl Grendlsen is a model for Necromunda, with rules and fluff inside its blister pack. »
2nd edition Warhammer 40k box set came with a large set of "2-D" ruined buildings that resembled theatrical decoration more than model dioramas. I pulled some out of the basement for a photoshoot to see if they are viable on the tabletop almost a quarter of a century later... »