Cut a hole under the wall for the cold sink and entrance. Put several blocks along one wall as a sleeping platform. The last block must initially be larger than the hole. Place the block on top of the igloo, then from inside, shape and wiggle it to slot exactly into the hole. Hot air from your body rises and is trapped inside the dome.
Cold air falls into the sink and flows away to the outside.
It is essential to cut ventilation holes in the walls with an ice ax. With warmth inside the igloo, the surface of the walls will melt and freeze over, to form a smooth, airtight ice surface. The roof over entrance tunnel prevents snow from blowing into the igloo.