First I cut various bits off the models and replaced them with green stuff moulded to look chaotic, then I undercoated them in Chaos Black and dry brushed them with Shadow Grey. After that I painted the chaos bits with Lich Purple mixed with Skull White, the guns with Boltgun Metal and the shoulder pads with Brazen Brass.
The base is first painted with Graveyard Earth. Then I applied a coat of watered down PVA glue and then poured sand onto them. Once that had dried I applied another coat of watered down PVA to fix the sand. The trick is to find the right colour of sand and then get lots of it so you never run out.