He once prepared for his oral examinations by opening a fresh notebook titled "NOTEBOOK OF THINGS I DON’T KNOW ABOUT." In it "he reorganized his knowledge. He worked for weeks at disassembling each branch of physics, oiling the parts, and putting them back together, looking all the while for the raw edges and inconsistencies. He tried to find the essential kernels of each subject.”
-- On the Feynman's notebook technique, Open Culture
-- Category Theory in Context, JHU Math http://www.math.jhu.edu/~eriehl/context.pdf
A friend of mine recently asked for help with getting started. It was very hard for him to overcome initial uncertainty: how do you know you’re sufficiently prepared to start filling your note archive? Which categories should you prepare before you begin?
My answer surprised him and was dead simple: don’t. Don’t prepare. Don’t invent categories. Just let it come.
‒ "Why Categories for Your Note Archive are a Bad Idea", https://zettelkasten.de/posts/no-categories/