I just did a series of these with green stuff, if I have time later in the week I'll try to get some decent photos. Anyway, my technique was something like this:
- Roll green stuff into a ball, them smush it flat onto a 60mm base.
- Clean excess green stuff from the edges, and then use a wet fingertip followed by a wet paper towel (use Brawny or shop towels that won't fall apart and leave lint behind) to smooth out the fingerprints.
- Push the model's feet down into the green stuff to leave a good impression. Pull the model back out, set it aside. You may want to lay them in a row with the drying bases next to them so you don't have to worry about what feet fit what base later.
- Use sculpting tools to gouge the mud into whatever muddy shape you want. If you have the model stepping up onto a rise, punch bullet holes into it

This is also a good time to add any extra junk (rocks, heads, arms) that you want to jam into the mud.
- Paint with a dark brown (I use Gamblin burnt umber liquid acrylic?it's not model paint)
- Coat the whole thing with a Brown/Chestnut wash - I use Reaper Flesh wash.
- Drybrush on lighter shades of red and brown to get a good muddy color. In between coats, apply more coats of Chestnut wash, focusing on recesses.
- Finish off by applying thinned black ink to the deep cracks/crevices.
Make sure that the mud color works with the model paint jobs?you probably don't want red clay colors clashing with Dark Angels green, etc.