The idea is to plan your diorama carefully before proceeding. First place all the stuff on the wooden base just to make sure you like the scene. Then remove everything and smear the Polyfilla all over the place and shape it as you like. Then, let it dry but touch the surface once in while and when the top is dry but it is still smooth underneath, use the figures and equipment to create tracks and footprints in the snow. You can also reshape some area. Then set everything on it and let dry overnight. Once everything is fully dried, you can still shape it with sand paper or Dremmel tool.
I recommand using a special formula Polyfilla. It is made for deep holes and will not shrink or crack. I made this a while back ago and it is still as nice as it was originally. It staid white and didn't crack at all.
Have fun.
No matter where you go ....... there you are.