Here is how I'm making the backdrop for my Endor diorama. I'm using my redneck engineering camouflage technique. What you need are poster board, dark green, light green, brown and tan/white spray paints. You will also need sticks/dowel rods, tree branches and or artificial plants.
First I spray painted the poster board brown, after that was dry I placed the dowels and sticks vertically to simulate trees. Then I sprayed it all light green. After that was dry I used various plant limbs and artificial pine branches as stencils and painted over them in tan. Afterwards I sprayed a few places, like the top and bottom dark green, using the artificial pine as a mask. Then remove the sticks/dowels. Thats it, easy backdrop.