I threatened (promised) to write more about the index system I use in my Franklin planner. This will be a pretty short entry.
The index is simply a listing of the things I think I may need to refer back to in the future. So I selectively choose just a few things to list. The short entries refer me back to the daily page entries. The daily entries have much more detail.

Franklin provides a page at the beginning of each month’s set of pages that provides 44 lines to mark the things you expect a need to refer back to. I expect if you have need for more lines you could add in another sheet. I think I would have to evaluate whether I really would need to refer back to that many things.
My expectation is that I will keep the pages for a year after I use them. In the past I have kept them for several years but I never referred back to them more than just a few months afterward. So, about a year seems about right.
That’s it. Pretty straight forward.