Here's my pennie's worth:
1. Add about 1 cup of water to an old Cool Whip (or similar) container. The measurement is just to get you started. The key to using Celluclay successfully is to use as LITTTLE water as possible.
2. Add a couple tablespoons of white glue and mix thoroughly.
3. Add paint to give you a basis for your groundwork. I like Liquitex Acrylics for this. Any of the earth tones will get your started. Add only a small squirt. Liquitex is a high solids acrylic paint, so a little will go a long way. Mix thoroughly.
4. Now, add 2-3 handfuls of celluclay and begin to mix it all together (Because of the large amounts of dust, this should be accomplished out doors or under some type of ventilation.) If it appears you have too much celluclay and not enough liquid, you are at the right mix ratio. Work the liquid into all the celluclay, until you get the consistency of a giant spit wad. If it's like oatmeal, it's too wet--add more celluclay.
5. If you are applying this stuff to a wooden base, Make SURE you have completely sealed the board; otherwise, when it dries , your board will look like a smiley face--or worse, when it finishes warping. I've used some of the acrylic polyurethanes (Minwax,etc.) with some success. In addition, I use a piece of old vinyl window screen cut to shape and stapled in place to give the celluclay something to adhere to. If you are going to construct hills, mountains, etc., use styrofoam as your formers. I've found celluclay performs best on bases the thinner it's applied. If you are adamant about building-up with celluclay, do it in increments--DON'T try to put a mountain in all at once--there is some shrinkage as it dries.
6. Lastly--thank goodness--keep a small container of clean water sitting next to you as you work this stuff. As you spread it out, take a little water on your fingertip and use it to smooth the surface. In addition, I've found it easier to wear some latex or nitrile (if you have a potential latex allergy) gloves. When you have finished, or need to stop, put the unused celluclay in a sealable freezer bag and stick it in the refrigerator. As long as it's moist, the stuff will last almost indefinitely.
Hope this helps. Let me know how it goes.
Gip Winecoff