Hi, I tried this for several years.
The materials in these projects are : a thick cardboard, a styrene, plaster or putty.
First, you cut a cardboard in size then also a styrene. Stick them together with pva glue and carve the styrene in a way you wanted.
Apply the surface with a putty or plaster. That's a lot of fun to do it. Just make sure you have a good references of natural scenes; may be the details of your backyard. Natural materials are also interesting choices. Sands and gravels are applied on the surface for details.
I painted the bases with acrylic colours then applied the base with crushed dried leaves. The plants were from a local aquarium shop.
Hope this help!