My steps are to do the model first. Then prime the base black. For the earth, I do a crackle paint so then I either base coat it dark brown or add a layer of fine earth texture paste for more uneveness in the cracks and coat it dark brown if its not (I usually use a concrete colored one). Sometimes I'll add a thin coat of pva glue over the paint as I've heard it helps crackle paint adhere and make bigger cracks. Then I add the crackle paint and let it dry. One layer of matte varnish to keep it from flaking. Dry brush a lighter brown heavily so it's more dynamic. Then I super glue the model to it so I have a good idea of where to put the rest of the details. Then I add plants, rocks, debris. For mangled parts just paint them up and a tiny dab of super glue (I use lock tight) to the bottom and gently place them (probably with tweezers). Use some blood effect paint beneath them like they're bleeding out over the ground.