Okay, here's the deal. You don't HAVE to do this step, or the step above that describes scoring the backing paper of the adhesive. You can just trip the rest of the bookcloth to the size of the bookboard and fold it over. These extra steps all detail HEMMING the cloth to make the edge prettier (since paper-backed bookcloth has an ugly edge when trimmed). Hemming the top and bottom is easy (you already did most of the work with the scoring). Hemming the right edge is harder, since you need to make sure it is exactly parallel to the left edge while also getting the correct length so there's no overhang (or underhang). To do this, fold the bookboard over the remaining section of bookcloth (WITHOUT exposing the adhesive first), and hold it tightly in place with the heel of your hand. Use your fingers to hold a straightedge against the edge of the bookboard, and then score the backing paper. This takes a few tries before you get the hang of it.