As some of the others have said, don't use celluclay. Primary reason I have for not using it is that it cracks and lifts from the base when it dries.
Rather, try using a product called Sculpt-A-Mold. A 2 or 3 pound bag goes for about $5 and lasts quite a while. Mix the Sculpt-A-Mold as the bag states, but use 25% white glue and 75% water. The stuff will dry quickly, so work it in layers.
For color there are two options, both work well. When mixing, try mixing in some acrylic paint, such as burnt or raw umber. Or, using the same colors, paint it when it's dry. I find that painting with an airbrush works better than brush painting.
That's the basics. You can play around with mixing in some find sand or coffee grounds to get texture.
SOme other things to consider. When the ground work is down and still wet, try using some old, dried plant roots to simulate trees, roots, logs, etc.