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Glue the endpapers to the sketchbook signatures.
On one of the endpapers, coat approximately ½” of the spine-side edge of the endpaper with glue. Fold the cover over (essentially closing the sketchbook), to align the glued portion of the endpaper with the last page of signature paper. Then flip the book over and glue the other edge of the endpaper to the nearest page of signature paper. Once all the gluing is finished, lay a few heavy books on top of the sketchbook so everything can dry without warping or curling.