Learn Anything In Four Steps With The Feynman Technique
This skips over a lot of work between 1 and 2... but I love how teaching it, or pretending to, gets you
Pick a topic you want to understand and start studying it. Write down everything you know about the topic on a notebook page, and add to that page every time you learn something new about it.
Pretend to teach your topic to a classroom. Make sure you're able to explain the topic in simple terms.
Go back to the books when you get stuck. The gaps in your knowledge should be obvious. Revisit problem areas until you can explain the topic fully.
Simplify and use analogies. Repeat the process while simplifying your language and connecting facts with analogies to help strengthen your understanding.