Here is the finished progress on the moss covered small pyramids and stone obelisk terrain. They were very easy to make.
Construction: I used some left over polystyrene to make the pieces for the obelisk piece. A little bit of work with the hot wire cutter to make the pieces in the shapes that I wanted and some wood glue. For the small pyramids, it was even easier to construct. My wife opened a box of something she had purchased. She said, "Do you want these styrofoam pieces?" I said, "Yes, I do!" They were perfectly square. So, I used the wire cutter to make a small square to top off each piece and glued them together with wood glue.
Painting: Everything got a base coat in the black Interior Home Wall paint. This was because spray painting will eat the foam. It is important to get a good seal with that base coat.
After the black, I mixed sand with PVA glue (watered down a bit). I painted this mixture over the each piece entirely to add texture. After the texture, I painted them brown. I wasn't worried about getting complete coverage. The purpose of the layering is to add variation and depth in the colors.
After the brown, I dry bushed codex grey followed by the same mixed with white. Continue mixing in more white with each successive layer of drybrushing until you are satisfied. Get increasingly more 'dry' or 'light' in your drybrushing as you go.
Final touches: I watered down PVA glue and painted it on, adding flock to make moss growing on the stones. I do this in small sections rather than trying to do it all in one go. After this dries, I take them outside and spray on a clear coat of sealant. This will keep your moss from rubbing off so easily during game play.