Properly constructed, this poor man's igloo can be body-heated to above freezing on a 20-below day, higher if you light a candle.
Step One Build up snow to a depth of at least 8 inches and pack it down to make a floor.
Step Two Heap loose snow onto the floor. Piling the snow over a backpack or mound of branches will let you create a hollow, which hastens the excavation process, but it isn't necessary. Let the snow consolidate for an hour or more, until it is set up hard enough to form snowballs.
Step Three Tunnel through the mound at opposite ends to dig out the center efficiently, fill in the unused entrance, and crawl inside to shape the interior. Ideally, the quintze should be narrow at the foot end, with a bed long enough to lie down on, and just tall enough at the head end for you to sit up. The walls and roof need to be at least a foot thick (check this with a stick).
Step Four Poke out an air vent overhead and dig a well at the entrance for the cold air to settle into. Cut a snow block for a door. Glaze interior walls with a candle to prevent dripping.