It's a lot of Lua. Ugly, ugly Lua that involves finding a good snow spot, moving your units to that location, and adding a bunch of resources to that spot. Then, obliterating your starting location. LastSword and I are slowly working through rebuilding the Lua so that it isn't 50 pages long and terrible (which is how I wrote it) and then we'll be turning it over to everyone to use wherever they like. Just like everything else, if there's no code for it, you can't do it. There's only code to start your characters on certain terrain types, and not on others. For example, you can't actually start *on* Tundra. The Tundra start code starts you *near* Tundra instead. That's because that's how Firaxis coded it. And there is no Snow code in the game source.