building a diorama base

First of all you want to make sure that you seal that chunk of plywood with something that repels water or your going to have one hell of a warped board.I`m working on a plain plywood board right now and all I used was a gloss acrylic enamal brown paint to
stain the whole board,course mine is quite a bit smaller. To do a desert terrain or groundcover I use two different colours of sand from Micheals,a dark sand and a light sand . Start with the dark colour here and there then with the light colour fill in the holes and overlap.Cat litter also helps greatly,again sparingly. When you get your desired effect freeze it with whiteglue/water mix. I should add here that the sands and rocks are layed down dry. More can be added for depth and up into the vehicle roadwheels and tracks as you mount your vehicles. I do my groundcover this way because I like the natural look. Have fun

I use a slightly different technique.

I build up the contours of my landscape with Polycell, then use Cat Litter and sand etc. on the surface fixed with White Glue.

I used styrofoam and spread elmers glue around and put sand on it. Not that complicated. Heres a pic.

I use plywood most of the time , or you can get premade bases from hobby stores and craft places.