Hi Alex, as my mate Tony has posted I have always used tile adhesive, the powdered stuff you add water to.
I mix this to cement like consistancy then spread it roughly over the base and shape it to any contour you fancy, pushing in rocks as you go. When it's dry I paint it with ordinary white silk emulsion paint. When this is dry and the figures and armour ect are in position, I get some white polyfilla powder in a fine sieve and gently tap it over the finished model. This gives the effect of a fresh fall of snow.
For the wet mud, where there is no snow, I just paint where I want the wet with ordinary household varnish.
Regarding the placement of your tank try this:-
When the adhesive is still workable but not sloppy, get the tank and gently push it into the mixture and withdraw it carefully, or if permanent, leave the thing in!
If removing, this will leave a nicely moulded place for the tank to sit. Don't forget the track marks behind the tank too, same principle but just touch the adhesive with the tracks. If removing, wash the tracks right away in water to clean them. Luckily I have a spare length of track specifically for this job....
A tip when painting the track marks white. Use a brush with very little paint on it and go along, not across, the track marks. This gives a nice hit and miss effect to the marks.
If you fancy it, have a look in my gallery at the snowy bits or click my picture and click on 'My started threads' and root out more snowy bits!
Folks have their own pet way of doing snow, so expect plenty of choice. I suggest you have a go at a few and see which suits you best...
I hope you find the above of some use to you.
Cheers,
Ron