Before we get started, here is a drawing that can be referred to if necessary, showing the direction we will be headed, structurally. This drawing also serves to illustrate the three main components of this treatment, the inner hinge, outer hinge, and paper surround.

The outer hinge is folded double on itself and glued together, then it is glued to the inside of the back board of the book's cover. The orientation here is the view of the book from the bottom, assuming the addition is being installed at the back of the book.

The inner hinge contains the addition but is not affixed directly to it. The addition is shielded by a paper surround, and it is this wrapping to which the inner hinge is actually glued.

And as can be seen, the structure is held together at its hinge point by stitching.