Two pieces of 20 point folder stock are cut.

The width of one piece is equal to W, the book's width. (This is the narrower piece in the image; we will call it "Piece A.")

The width of the other piece is equal to L, the length of the book.

Grain orientation of both pieces is grain short.

As can be seen, the direction we're headed is toward a structure with a center panel (the overlap) and four flaps. The minimum lengths of the flaps are given in the illustration. (Again, these are the minimum measurements; excessive is fine here.)

Whether or not you measured the thickness of the book in the previous step, an approximate (T) measurement will do for now, as long as the total isn't too small. For your final measurement, you will use the instructions shown a few steps ahead.

With "Piece A" on top, the two pieces are aligned squarely and glued together at the overlap, using PVA or PVA mixture. They are placed under weight, with the assistance of a flat board if necessary, and allowed to dry for an hour or so, or preferably overnight.