The traditional method of igloo building adds the blocks in a continuous upward sloping spiral. That works well, especially if you have an assistant helping you. But when I’m working alone I prefer the concentric ring method shown in this video. Regardless of the method, I try for the igloo roof shape to be about half-way between a true hemisphere and a cone. I believe the technical term for this shape might be a catenary arch or a parabolic arch. I use the string when teaching first-timers to cut blocks at the correct angles, but once you’ve made a couple of igloos you’ll be able to eyeball the cuts just fine without it.