First, I cut a piece of rigid foam or use a piece of an old mannikin.
Next, I sand or scrape it to the shape I want, making sure to sand off all of the slick areas where mold release was used.
Next, I mix 50/50 Durabond powdered drywall plaster and Vermiculite ( I get it at plant stores or nurseries)
Next, I cover the entire foam piece with it, except the bottom, and let it set up for a few minutes.
Once it has set and firmed up a little, I use a damp paper towel and smooth down some of the pointy looking areas.
After this has set up ( about 20 minutes ), I mix about 5 parts water to 1 part powdered Tempra paint.
Very simple and saves a lot of money if you need to add some rocks to your habitat base! I attach them with hot glue.