Welcome to the site! I make mud by mixing baby powder with an acrylic dark brown paint and a little static grass. I usually make the stuff so it's the consistency of slightly lumpy by still runny oatmeal, then apply it with an old brush by dabbing it on to give it a patchy, splattered look. Once it's on the kit, you can apply washes and drybrush as desired.
There are many, many other methods, but this is my personal favorite.
Hope this helps!