I got this idea from a friend who made her own planner from a used book. I cut out the inside of a book that I honestly would never read to use the hard cover. Then I took 32 sheets of Cardstock paper, folded each sheet in half, bound them together by sewing; then secured my sewing by gluing with Mod Podge. Then I glued the book into the cover.
I'm going to try this again to write a tutorial.