@chrisjolliffe Start by doing a very cursory overview of what you've got (e.g. photos, photo negatives, correspondence, flyers, etc.)
Take notes about how things are organized (if there is any organization). Materials might be organized by events, or type of material, or something else. If there is an organization system in place, it might make sense to maintain and expand upon it.
If there is absolutely no discernible organization system, you'll want to create something.
Excel (or Google Sheets) is fine to get started.
Let's say you've got a bunch of photo prints, and let's say that it makes the most sense to organize them by approx. date.
You might put all of the photos from, e.g. 1950–1959 in one folder (acid free manila folders are best). You would then label the folder something like "Photos 1950–1959" in pencil.
In your spreadsheet, you'd type "Folder: Photos 1950–1959" into column A and write the location of that folder in column B.