Step 2: Cutting the cardboard
Here we're going to make a cardboard frame to hold the mirror in place. Before you do anything, though, we want to make sure your cardboard can fit into the hot shoe connector well. Cut an 11/16" wide strip from your cardboard source and try putting it in your hot shoe. If it slides in snugly, great! If it's loose, you might be able to fix it by taping or gluing several layers together. If it's too big, you probably need a different type of cardboard (although you might be able to use it for parts of it).
I intended originally to cut my cardboard into one piece that could be folded into the final shape, but I ended up doing it in two parts. I think this worked out to be easier, and I recommend it. All of the dimensions worked for my mirrors and my camera, but you may need to adjust it for your materials and equipment.
Here's a rough diagram of how my carboard pieces were cut out. Depending on your camera and the size of your mirrors, your measurements may be different. Experiment. I've shown dotted lines where I left extra space to cut things down later. Easier to remove cardboard than to have to start all over.