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
During the April 16-17, 2016 Weekend at Games Workshop stores you had a chance to get a special edition remake of the original 1985 Space Marine model. The new version is substantially beefier than the original and the difference is much more striking than the one between the original Mk. 4 marines and their 'Betrayal at Calth' reimagining »