All my terrain and miniature bases get the "trampled grass" look. Here is how to make it
Grass around a bunker that has seen a lot of fighting is going to be trampled and muddy all right
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
Spread a thick layer of un-thoroughly mixed gypsum plaster to the patches of mud
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 :-)
Paint some areas of grass with diluted earthly brown (as Humbrol 113 Rust) and matt red paints
The patches of mud are painted with an irregular mix of dark green and brown
The mud is subsequently painted with a semi-glossy varnish
Originally designed to fill gaps in failed finecast miniatures it does a really bad job at that. However it works great for adding texture to models and fixing problems with the "Chinese import" quality Games Workshop terrain »
To have some fun, show you where my approach to wargaming comes from and share my way of using polymer glue to prevent cardboard warping I put together this short video »