Diorama that I built recently for the IPMS National Competition. More information and pictures can be found here:
Progress page: [link]
Completed page: [link]
I started off with blocks of styrofoam that were cut. After gluing them together, plaster cloth was used to cover the styrofoam. After a few days of drying, I used sculptamold which is similar to cellyclay but has a claim that it doesn't shrink. A thin layer was placed onto the hardened plaster cloth. Gray paint was sprayed over the surface, then I used dirt I found outside. White glue was applied to the base, and the dirt was sifted over a strainer to get a finer grain over the base.