How to Write a Commonplace Book

I do things quite a bit differently because I write my notes down as I'm reading the book. So at the back of the book, and where did I get this? It was from a podcast with Tim Ferriss and Maria Popova,. And they both talked about how they do these indexes at the back. But as you run across something that strikes you, you write down the topic of that quote, underline it, and then put the page number to it.