Step 1: Start with 1mm thick MDF board cut to 600x600mm. Paint the underside Black and seal with varnish. Top side using Polystyrene sheet cut hill sections. From a mould I had kicking about I made some paving for a brick road out of PVA glue.
Step 2: Lots of PVA glue.... and more. Glued the rock sections down and made the glue for the water section.