Using fabric to cover plain diaries and notebooks is easy. The method is basically the same as covering school books. The only diffence being that the inside of the fabric needs to be sprayed with adhesive and rather than using sticky tape on the inside cover to hold the paper/fabric in place, use craft glue instead.
Once that's done, glue the first and last pages of the diary to the front and back covers to hide what you've done. Purists may wish to use fraystopper where the fabric has been cut at the top and bottom of the spine but I never bother - and I've never had a problem.
Small Things Simple Pleasures