Before you make any sort of mark of the paper (notes, underlining, highlighting) read the chapter/assigned reading. It’s usually not applicable to a textbook (but very good for literature books), but at this point I also look up any vocab words I don’t know (sometimes I’ll write these in the margins if I think I won’t remember the meaning in class). My goal on this first readthrough is to fully understand the content. That way, when I’m taking notes for real, I know what’s coming up so my notes can be better organized. Making sure I understand things also means I can put it in my own words, instead of copying sentences I don’t understand.
The next step annotating the book. I have a color coding scheme that I use across all of my notes. Pink is also a variable definition, red is an example problem, etc. When I’m taking notes at this stage, it’s so that I can quickly go back through the textbook and figure out what the main ideas are. I do this inline with the text, so there’ll be something like “Nernst Equation—->” so it’s easy to find it later on. With English books, I’ll also have a summary at the beginning of this chapter telling me what happened in the chapter.
It’s usually pretty easy to figure out what’s happening in a given chapter of a textbook, so I don’t do this. Depending on the complexity of the subject / how lazy I’m feeling, this is where I’ll end. I have some classes that assign fuck tons of work, so I just annotate my textbook well then work through the problem set. For things that are more conceptual (and I’ll want notes to review from at the end of the year for finals) I’ll take separate notes on paper. This time, I read the textbook through for the 3rd time. This time is pretty fast, mostly just writing down what I wrote in the margins the first time.
Generally by this stage everything is in my own words, but even if it’s not, I know that I understand it (or perhaps more importantly, I know what I really don’t understand by this point and then I make a big mark to go ask my professor in lecture/office hours). Also look at the beginning / end of the chapter for a summary of important points and pay attention to what the teacher stresses in class. Sometimes there’s stuff in your reading that’s not going to come up again. It’s worth reading for better understanding, but don’t bother taking notes on it.