1 Collect a few dozen half-gallon plastic containers — milk jugs with their top halves cut off work just fine. “The more you can get,” says Alvestad, “the faster your igloo will rise.”
2 Wait for a good strong cold spell that’s forecast to last at least a week. “It’s a bit of a gamble. You are totally dependent on the weather.”
3 Fill up the containers with water and leave them outside overnight to freeze.
4 Once they’re frozen, pop the ice bricks out of their containers (you may have to take them inside for a few minutes first to allow them to melt enough to come loose). Create a circle of ice bricks, cementing one brick to another using wet snow as mortar. “Don’t forget to leave a gap for an entry tunnel.”
5 Start building upward by adding a second layer of bricks, then a third, and so on, tilting the bricks inward a bit as you go higher (this may require you to hold the bricks in place for a minute or two to let the mortar freeze). Over a period of days — how long depends on how many bricks you can freeze each night — continue building upward and inward until you’ve closed the igloo’s roof.
6 Use another batch of bricks to build an entry tunnel wide and tall enough to crawl through.
7 When your igloo is complete, spray it with water to give it a good icy outer shell.