The first thing my son did was draw out the shape and size of the igloo with his feet. We brought plastic toy boxes, which the boys filled and refilled to make snow bricks.
Their persistence was incredible!
Franciful Arts sent a great link on how the Inuit build an igloo. The little movie was made in 1949, and is fascinating. The narrator calls the Inuit "eskimos," as we used to when we were little, but this is considered offensive by some, because it may be a denigrating name given by foreigners, and may mean "eaters of raw meat." Inuit is the name the people have for themselves. It means "the human beings."
Thank you, Franciful Arts! We'll try it this way next time!