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
There is a fine balance in making a tabletop hill the right mix of a gaming token, a unit display stand and an eye-candy. When you let it slide in one of the directions it will lose its unique capabilities of elevating your wargaming experience to the next (excuse the pun) layer. »
After many years here comes a new card terrain set for Warhammer 40k Cityfight/Sector Imperialis setting. It's extremely wobbly but can be fixed and I show you how. »