https://thoughtcatalog.com/ryan-holiday/2013/12/the-notecard-system-the-key-for-remembering-organizing-and-using-everything-you-read/

This was the original post which Holiday copied to his own site at: https://ryanholiday.net/the-notecard-system-the-key-for-remembering-organizing-and-using-everything-you-read/

Cross reference notes there.

reply to u/ihaveascone at https://www.reddit.com/r/Zettelkasten/comments/14v2ohz/converting_commonplace_books/

Don't convert unless you absolutely need to, it will be a lot of soul-crushing make work. Since some of your practice already looks like Ross Ashby's system, why not just continue what you've been doing all along and start a physical index card-based index for your commonplaces? (As opposed to a more classical Lockian index.) As you browse your commonplaces create index cards for topics you find and write down the associated book/page numbers. Over time you'll more quickly make your commonplace books more valuable while still continuing on as you always have without skipping much a beat or attempting to convert over your entire system. Alternately you could do a paper notebook with a digital index too. I came across https://www.indxd.ink, a digital, web-based index tool for your analog notebooks. Ostensibly allows one to digitally index their paper notebooks (page numbers optional). It emails you weekly text updates, so you've got a back up of your data if the site/service disappears. This could potentially be used by those who have analog commonplace/zettelkasten practices, but want the digital search and some back up of their system.

u/zleonska in discussing their paper notebook commonplace practice reports that finding their material within multiple notebooks isn't difficult but that, like W. Ross Ashby, they use index cards to index their commonplaces.