Step 3: Labeling and perforating the pieces
I opted for the rivet joinery method, as this automatically provides overlapping tabs and holes in the appropriate places on each panel. I spent a while fine-tuning the exact hole diameter and tab width, using scraps of cardboard to roughly test how much stress different arrangements could withstand.
123D does a great job of labeling all of the 2D pieces it generates, with each panel being automatically labeled with its own number and the numbers of the pieces that each edge should be touching. What's more, you can choose to have the precise angle of each fold labeled, letting you know whether the fold is a mountain or a valley and just how far it should be bent.