This has been done plenty of times before, but here's my own take on it.
I had an unused diary which was falling apart, so I used a scalpel to cut a hole in the pages (as demonstrated e.g. here) and rebound it in cloth (as demonstrated e.g. here). The ribbon (to assist in removing the kindle from the case) is fixed in by stapling to some of the rear pages, and then threaded through the top of the case.