The enclosure is folded into fully closed position. (It works well to do this with the book in it.) Marks are made as shown, in the corners at the base of the tab and in line with the width of the opposing (right) flap. The first two marks are for the slit into which the tab will tuck. As indicated by the red arrow, the cut is made a little longer than the width of the tab.

Note:The end of the top flap should fall just short of the mark on the right and thus be hidden from view by the two outermost (left and right) flaps. If it extends beyond this mark and is visible, it needs to be trimmed back.