I add as much as I need to get good color saturation. That, of course, depends on the amount I'm mixing. Here's what I do: put some Celluclay into a container. Squeeze in a good size dolop of white glue, e.g. Elmer's, pour in a bit of paint, toss in some railroad ballast for added texture and add some water. Mix it to gether with my fingers. If it's too thick, I'll add a few drops more water. If the color saturation too weak, I'll add more paint. If it's too watery, I'll toss in more Celluclay. The end result should look like thick oatmeal and you should not be able to squeeze out water.
Before applying it to your wood base. drill a bunch of holes in the wood. This will give the mixture something to bite into and be less likely to rise up. The glue helps this, too. When dry, paint, add washes and highlights. Apply your favorite grass product where you want.
If you need to build up any significant elevation, use layers of styrofoam so you never have more that about 1/4 to 1/2 inch of Celluclay.