I like the water activated snow flocking as well as the ground up styrofoam kind that you glue on.
The water activated kind is better if you want to sprinkle your entire scene. You just mist the whole thing with water and then apply the snow with a flour sifter. It gives a very nice effect, but you can't build up much depth with it. It looks like new snow that fell when it was already COLD outside. Sort of frost-like.
For the styrofoam kind, just use white glue like Elmers. You can also glue it to white polyester batting, like George said, if you want to make it look deeper, or you can pre-sculpt you snowdrifts from white foam.
I will often mix the two kinds of snow together because it's easy to apply a light dusting of water-activated snow on top of the foam kind.
This mount used both kinds.

The light dusting on the rocks and leaves is the water-activated kind from WASCO.
The main ground cover snow is the styrofoam kind glued over white-painted blue foam.