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
Peter Jackson's crew made a great work portraying the Mines of Moria in their movies. If you want to see something eerie similar in real life, you can visit one of the original 12 Unesco World Heritage Sites, namely the Wieliczka salt mine near Krakow, Poland »
Many players magnetise their models to have an option to change their weapons. But even if you're a hobbyist keeping faithful to one model configuration, you may benefit from magnetising some parts »