All my terrain and miniature bases get the "trampled grass" look. Here is how to make it
1. Glue some static grass, leaving irregular patches of flat areas that will become mud. Press the grass with something flat like a painting cup bottom immediately
Don't use PVA glue on cardboard: it will warp the base. Read more on what glue to use
2. Spread a thick layer of un-thoroughly mixed gypsum plaster to the patches of mud
3. Paint the whole grass with heavily diluted dark green paint. I use Humbrol 105 Marine Green mixed with turpentine — it will protect your models against bugs as well :-)
4. Paint some areas of grass with diluted earthly brown (as Humbrol 113 Rust) and matt red paints
5. The patches of mud are painted with an irregular mix of dark green and brown
6. The mud is subsequently painted with a semi-glossy varnish