Feynman Method

If you really want to understand something, teach it.

Dan Sheffler in Going From Reading to Notes:

While reading a book or article, I make a note every time I have a substantial thought or observation. I begin by writing the page number I am currently on, then the thought. As much as possible, I try to write in my own words, critically evaluating what I am reading rather than merely echoing. If, however, I think that I will need to quote an author explicitly, I copy down the quote making sure to place directly quoted material in quotation marks. I then double check the quote for accuracy. All of this should take a minimum of effort and organization while still maintaining clarity and depth of thought.

See what Dan is saying: Do not echo.

This is why people recommend you don't look at a text when you are trying to summarize or reflect on it. You're trying to test your understanding, and looking at the text is cheating.

As a side thought, there's no reason that you shouldn't do this multiple times with the same book. In fact, this is a way to test how your understanding improves. Maybe you'll have a lot of trouble doing this your first try. Then, after reading a book more deeply and coming back in a few weeks, you'll find it's a lot easier to talk about the ideas in a book.

I do this naturally by writing essays on what I read—I find that I am incapable of writing essays if I don't understand the material I just read.