Somewhere in the modelling threads, I posted part one of a Diorama Guide (still haven't finished part two!!) which gives some ideas for various surfaces etc. It was a couple of years ago, and sorry, I don't know how to advise on how to search for it.
As WW was/is basically flat, meadow grass, it would probably be easier and more effective to use a 'hairy mat' grass sheet, available in rolls as already mentioned, and this is, as I remember, included in the Guide.