First off, you need to stain and seal the wood with a good varnish or eurathane. This will keep the wood from warping by absorbing any water from the process. If you are looking to just do some basic groundwork of dirt and grass, all you need to do is use some white glue (Elmer's) and cover the base with it, then sprinkle on some dirt, add static grass, and paint. For more relief, you can use products like celluclay (a paper mache-like product), Durhams' Water Putty, pieces of cardboard covered with plaster, etc. to build up the relief for the ground. Then cover it as above with dirt, static grass, paint, etc.
Hope that helps. Good luck.
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