1. Recondition the leather to prevent tearing in the next step
2. Slowly lift the leather, if possible use heat to help and to avoid distorting the leather.
3. Scrape off corrosion underneath and on the backside of the leather. Let the leather flatten out between two books over a week.
4. Reapply leather with shillac, or a very mild glue
5. Recondition the leather again.
It’s time intensive but worth it.