Matt, There are several YouTube videos on winterizing terrain.
You're better off watching them. It's easier to follow along watching someone actually doing it.
But basically
Cut and sand down MDF
Gather up cheap trees and some WS armatures.
Paint and flock the trees for winter without snow.
Hot glue them to the bases and spread your Dark Brown ground mixture all around the base.
Put down clumps of static grass around the edges and drybrush them a dark brown.
mix a paste of WS Soft Flake Snow.
1 cup Snow, 1/4 cup PVA, 1/4 Water
Frost the whole thing with white spray paint
Apply the snow paste wherever you want it.
Sprinkle Snow Flake on the spread paste
Seal with Dulcote.
Sprinkle on more Soft Flake snow.
Tap the excess off.
DONE