First I cut the foam core into 20"X20" panels. I then labeled each panel according to it's location on the box to keep myself from getting confused.
After I labeled my 4 sides I measured a 1" thick margin where each joint connected. Then I laid out the panels according to where they connected to the 'back' panel. Imagine the sides and top folding up at each joint and you can visualize how the box will connect.