I reckon that the best way to make it is to have a thin foam board base (for strength - you can even buy foam boards with interlocking edges, which is great), glue a thicker (like an inch or an inch and a half think) sheet of cheap polystyrene over it, to fit. Then cut into the polystyrene and use plaster or acrylic grout to smooth it out. I used bathroom filler in a squeeze stick to make little hillocks or grassy banks. And you can glue on slivers of polystyrene too. Then sand glued on with PVA and then given a dilute PVA wash after. Which you can paint of course.
Boards made like this are incredibly light and strong. The only weak point is the edge of the weaker polystyrene. I dealt with that by cutting thin strips of card, folding them in half lengthways, giving them an 'L' shape end-on, and gluing these strips along all the exposed edges. I used pins to keep them in place until they dried. This is before the sand stage.