I have build the base board for my layout from 6 mm plywood and some strip wood, about 18 mm x 40 mm. The whole board has been build in one piece of 180 cm x 36 cm. After defining position of the small stream, I prepared the extruded polystyrene (yellow foam, in this case) and glued it to the bord.
I drew the planed positiones of structures, hills and track onto the foam and started to carve some landscape out of the planes. After that I added some additional layers of foam for the hill on the left of the layout.