On Isaac Newton's iteration method to self-learn geometry:

"He bought Descartes' Geometry and read it by himself .. when he was got over 2 or 3 pages he could understand no farther, than he began again and got 3 or 4 pages father till he came to another difficult place, than he began again and advanced farther and continued so doing till he made himself master of the whole without having the least light or instruction from anybody" (King's Cam.,Keynes MS 130.10,fol. 2/v/)

Numerous anecdotes exist on studying strategies, like the Feynman method explained here "If you can’t, out loud or on paper, explain the idea without confusion or contradiction, stop and figure it out right there". There's some books that model that method, like Gilbert Strang's Calculus has you reciting back the entire chapter you just read.

If you ignore the copious amounts of marketing on his site, Cal Newport has some other interesting anecdotes on studying, such as how he was able to get the best grade in his Discrete Mathematics class, and the rest of the site is full of advice on studying, how to schedule yourself and deliberate practice.

My personal advice is to first always get the errata for what you're reading, even course notes sometimes have errata on the author's page, and always take something a little harder than your skill level so then it becomes a research exercise backfilling all the requirements. For example many people want to relearn math they forgot, so they start working through some enormous 1000+ page pre-calculus book and lose interest after the first few chapters. Instead make the goal to learn calculus, and start there. The same goes for learning algorithms, make the goal you want to solve problems in competitive programming or you want to build something impossible and need to learn how to make it possible, now it's a research project to learning fundamental algorithm design techniques that will keep you interested.