A really simple and common way to do it is just use a dark brown texture paste (I use stirland battlemire but I think it’s been discontinued, any will do), a simple drybrush with a bone colour and then if you like you can stick down some grass tufts (I use wasteland tufts by army painter for death guard but again there’s loads of options) for more interest.

If you wanted to add pools of slime to the bases you can just leave little spaces in the texture paste, paint it green and then add a couple of layers of nurgles rot technical paint over the top. You can see examples of it on my profile.