to about a half-inch long so that it will easily tuck down into the inside of the spine. apply glue to the inside of this small piece of paper and push into the spine, using a tool to press it against the inside of the spine if the area is too small for your finger to fit. the handle of a glue-spreader is quite useful here. repeat for the other end of the spine. if you cut the paper smaller than the full width of the inside spine, you will finish up with an area of the old cover still showing through, between the spine and the front/back cover, once 