Measure the width of the spine, then mark and score two lines in the cover to form a square-section U-channel. Use cotton gloves to avoid fingerprints marking the cover. Note the use of the ordinary desert knife for scoring. A Stanley-type knife is too sharp for this job. Also, only light pressure is needed with the scoring knife, and the blunt side of the blade is used.
Make sure that the book block fits snugly inside the channel. When two pieces of card are used like this to form a composite cover, the rear cover is stuck on afterwards.