We used Supermoss Moss Mat that we picked up at Hobby Lobby. They are thin, flat sheets that can be cut with scissors. Lay down some newspaper before you start for easier clean up.
Step 1. Cut the moss 1" larger than your displays.
Step 2: Fold the moss sheet around to the back of the board. Pull the moss sheet tightly and glue it down.
Step 3: Go back along with edge of the moss with another bead of glue to secure the edges.
Step 4: For the corners, trim the excess moss and glue the edges of the corners to the display.
And that's it! You'll want to give it a few taps on the table to shed some of the loose moss. I also recommend wrapping these in bubble wrap when you are packing for a show to protect them.
(Jess created these, I was merely the art director. Add display and prop guy to his job title!)
You can fill little bowls with Styrofoam and glue some of the moss on top for coordinating displays. Just make sure to balance all this mossy goodness with other colors and textures.