Below is my step-by-step guide for making a mud environment for WWI or later battlefields.
I used foam insulation on a base of wood I got at a craft store. The insulation allows me to build up the mound of dirt without having to use too much scullpy clay. Attach the insulation to the wood with a couple nails. No need to be careful since it will all be covered with scullpy clay. I also cut out a crater in the center to use a a mud/shell hole later on.
Next I used celluclay and some small rocks to make my mound of "dirt". Don't smooth the clay out, but leave it lumpy. I also buried a piece of balsa wood to be used later as a sign post. The celluclay should be mixed with water to the consistency of lumpy oatmeal. The clay can take up to three days to dry depending on the amount of water used, so I use a fan to speed the drying process.
Next spray paint with walnut and cherry colors that I picked up at Michael's Craft store for $4. The paints would be terrible for wood as they go on very unevenly, but since you are looking for that non-uniform look, they work very well as simulating mud.
Next I filled in the hole with Scullpy clay. Scullpy clay doesn't harden unless you heat it, so why use it? It's great for sticking things into.
In the next shots I painted the scullpy, poured "real water" after the paint dried and then I pushed a helmet into the "mud"
Lastly, I added a road sign and some splinters and some grass on the top of one mound for contrast. The grass is held in place with craft glue that sprays onto the surface
And there you have it. Muddy terrain for about $6 and you can make it two days.