Your glasses should not fog up unless you wear a neck gaiter or balaclava over your nose. If you put it over your mouth, leave a bit of room above your mouth open so your breath doesn't shoot straight up onto your glasses. It might take a bit of experimentation to get it just right.
Here's my article on Cold Feet ... what I wear to keep my feet warm:
http://www.machka.net/whatworks/coldfeet.htm