Cut out the pages with a sharp knife, except for the end pages. These will add cushioning and stability.
Cut the spine in half, then trim to make your desired size, plus 1/4″ for a seam. Sew a ribbon on each side, then sew the middle edges together. Figure 1 shows the spine from the outside, Figure 2 shows the spine from the inside.
Glue 3/8″ elastic in C-shapes for each corner (Figure 3). Cut and glue paper over the inside tabs for greater stability. Glue a ribbon over the inside of the spine, if desired.