You can now remove the tape and cut the tails off the threads.
 

Prepare the cover

I used some spare A4 covers, as I have an ample supply of these. However, it does mean making up a composite cover by sticking a separate back on after fusing the front cover with the book block. You may wish to obtain a larger sheet of suitable card stock from which you can make the cover in one piece.
 

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.