First, you start with a bunch of polystyrene: take your hot knife and cut out some pieces of foam that will roughly be the size of your hills.

Next, make all of the precise cuts - making sure to create many different levels, so that when you dry brush there will be lots of highlights.

Then glue the two pieces of polystyrene together, and weigh down as it dries so that no warping will take place.

Paint a dark gray, then drybrush a lighter gray over.

Add flock with watered down PVA glue.

Finished product: