First you will need to create some templates to mount your rubble onto.

Now mix up some plaster. Use a container that you can throw away, like my victim: the dunkin donuts cup. When mixing plaster always use luke warm water.

Always pour plaster mix into water, not vice versa. Always pour the mix in slowly. Do not stop pouring unil the water is saturated: meaning you get little "islands" of plaster that will not sink to the bottom; observe:

Next tape down some newspaper to create little rocky structures. This creates a base for the plaster to settle into.

Now add the thickening plaster to the paper structures you just made.

Wait for the plaster to dry a little more then "rough it up"

Don't throw away all of that extra plaster you didnt use! Instead "rough it up" in the cup and let it dry. When it's dry and chunky it will be useful for future projects.

Paint the rough a shade of gray and then flock the base. Let it dry for 20-30 minutes before shaking any excess flock loose.

Remove excess flock, drybrush, and paint details.