Not only adding tension to any landscape while offering troops some cover but also useful markers for exploded vehicles
1. Start with a proper base
2. 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
3. Cover the whole thing with a PVA glue and sprinkle with fine sand:
4. When it is all dry, shake off excessive sand
5. Spray it all with a black paint
6. Drybrush it all with a grey paint
7. Glue some static grass to the base's rim
8. Blend the line between grass and sand with some black paint
Check out also other texturing methods I reviewed and decide which one suits you best for a given project.