There's been a lot of discussion about this on HN. Here's my annotated list of links that should get you started:
0.) To get started not copying, try paraphrasing (writing down concepts in your own words.) and ranking statements you paraphrased on a scale of 1-10 (10 is most important). Do enough of this tedious work, and you'll start to >>summarize<< what you're reading, and get an intuitive sense of what's most important to summarize.
1.) Search HN for "zettlekasten" and watch the youtube video about it: https://www.youtube.com/watch?v=nPOI4f7yCag
This will help in understanding that note taking is a process and what's important emerges as part of this process. Expect some inefficiency.
Pro-tip: physical notecards are still the best for fewer than 5,000-10,000 cards.
2.) Here are some tips on the physical aspects of note-taking that helped me: 1.) use a few different colors, 2.) use a highlighter for important concepts, 3.) use the Cornell page layout (see #3 below), 4.) have one main concept per page/card so you can rearrange into a logical order, 5.) summarize key points at the bottom of each page, 6.)
3.) Try the Cornell method:
http://lsc.cornell.edu/notes.html
https://ubwp.buffalo.edu/ccvillage/wp-content/uploads/sites/...
layout explained: https://www.csuohio.edu/sites/default/files/Note%20Taking%20...
4.) You can fall into a rabbit hole of optimizing notes. At the end of the day, spending time studying is all that matters.
