This started with some scrap 1/4-inch plywood left over from a home repair project. It was close enough to 150mm deep that I was able to use it without having to do any extra cutting. With most of the scenery going to be below the track level, I flipped the platform over to form a box that will hold the foam.
The foam was also scraps saved from the trash bin. I ended up with a mixture of sizes and ended up fitting them together like a puzzle.
I do not have any pictures with the intermediate stages. My standard method is to glue the foam down with wood glue and let it dry for a few days. Next I use a large knife to roughly carve the ground into the kind of shape I want. Then I mix up a batch of plaster -- often more watery than it calls for -- and soak pieces of cheap paper towel in it and use them to cover the foam. I spoon in extra plaster to fill in the cracks and holes.
Once the plaster has dried, I use latex interior house paint as a base coat. I'm currently working on a gallon bucket of Cape Cod Gray. Once that has dried and everything looks fine, I use acrylic paint as a second base coat for the grass and dirt areas. After that has dried I glue down flock and ground foam.
The fence is made of toothpicks, paper strips, and a nail for the last post at the end of the module.