If you're anything like me, you'll hate the GW style "snow flock". Or, white static grass. It never looks like snow, so I use a different method. Its a bit longer winded but works much better in my experience.
What you need
Paints; Chaos Black, Charadon Granite, Adeptus Battlegrey, Astronomicon Grey
Flock - not needed, but adds to the base
Bicarbonate of Soda
No pictures here. Basically make your base how you wish and undercoat it. Here I've just done sand and painted it black. Makes for nice contrasts.
The first layer is to drybrush Charadon Granite all over the base. Be as heavy as you like.
Next layer you drybrush Adeptus Battlegrey
A final light drybrush of Astronomicon Grey
Step Five - Optional
You can add flock to the base at this point, or you can leave it off. The choice is yours. I usually leave it off - but only if I'm doing the whole base snowy.
Take some PVA glue, and some Bicarbonate of Soda, mix with water until you get something that looks akin to the below;
It should be thick enough that it sticks to the brush, and doesn't run off it. (Note; It be best if you do this stage with an old brush you don't care about, as this stuff could ruin a good brush).
You want to apply the mixture quite heavily. It often shrinks as it dries. Cover as much as the base as you like. Then, again an optional step is to sprinkle pure Bicarbonate over the snow, to resemble fresh snow. Then leave to dry. Base complete.