2. Cutting out window and door openings
The plans were then drawn on 3/16th gatorboard. It's like foamcore that you use for school projects, except gatorboard's outer surface is coated in resin, so it's very rigid, light and durable and can be cut with an X-acto knife.
Here I am squaring up all the openings with the trusty 220 grit sandpaper glued onto a block of wood.