Get a piece of paper and a pen, then write stuff down. Analyze why would you like to even take notes; don't focus on pretty notes, at least if your goal is understanding.

Think about it, rethink it other day, talk about it with someone else...Don't expect some kind of recipe of "how to..." that will tell you the very secret to finally understand ideas.

When you are done with the book, use what you can recall and your notes to try to draw some conclusions. Those conclusion don't have to be perfect and definitive, they're just one step towards working on those ideas. You can always revisit the work later.

And most importantly, don't mind doing it perfect from the beginning, you will curate the process as you go. You have some books about how reading books too if that's very important to you.