The daily cards and journal entries are obviously indexed by chronological date and then within tabbed sections by month and year.
The rest of the other cards with notes are given individual (decimal) numbers and and then are put into numerical order. These numbered cards are then indexed by putting related subject/topic/category words from them onto a separate index card which cross references either a dated card or the numbered card to which it corresponds. These index cards with topical words/phrases are then filed alphabetically into a tabbed alphabetical section (A-Z).
As an example with the card in this post, if I wanted to remember all the books I buy from Octavia's Bookshelf, then I'd create a card titled "Octavia's Bookshelf" and list the title along with the date 2024-08-13 and file it alphabetically within the "O" tab section of the index. Obviously this might be more useful if I had more extensive notes about the book or its purchase on the 2024-08-13 card. I did create a short journal card entry about the bookstore on 08-13 because it was the first time I visited the bookstore in it's new location and decor, so there are some scant notes about my impressions of that which are cross-indexed to that Octavia's Bookshelf card. Thus my Octavia's Bookshelf card has an entry with "The Book Title, 2024-08-13 (J)(R)" where the '(J)' indicates there's a separate journal entry for that day and the '(R)' indicates there's also a receipt filed next to that day's card.
I also created an "Author Card" with the author of the book's name, the title, publication date, etc. I included the purchase date and the reason why I was interested in the book. I'll use that same card to write notes on that particular book as I read it. These author cards are filed in a separate A-Z tabbed 'Bibliography' section for easily finding them as well. (I suppose I could just put them into the primary A-Z index, but I prefer having all the authors/books (I have thousands) in the same section.)
I also have a rolodex section of people filed alphabetically, so I can easily look up Steve and Sonia separately and see what I might have gotten them on prior birthdays as well as notes about potential future gift ideas. I had tickler cards with their names on them filed in early August and now that they're in my to do list, I've moved those cards to August 2025, ready for next year's reminder. Compared to a typical Future Log I don't do nearly as much writing and rewriting when migrating. I just migrate a card forward until it's done or I don't need it anymore.
If you've used a library card index before, the general idea is roughly the same, you're just cross-indexing more than books by title, author and subject. You can index by day, idea, project, or any other thing you like. My card index cabinet is really just a large personal database made out of paper and metal.
The secret isn't to index everything—just the things you either want to remember or know you'll want to look up later and use/re-use.