“[He] opened a fresh notebook. On the title page he wrote: NOTEBOOK OF THINGS I DON’T KNOW ABOUT. For the first but not last time he reorganized his knowledge. He worked for weeks at disassembling each branch of physics, oiling the parts, and putting them back together, looking all the while for the raw edges and inconsistencies. He tried to find the essential kernels of each subject.”
I always take notes in a notebook when I study, or are trying to learn something I don't understand. The first thing I do is to read everything once and make a note of everything I don't understand. The next step is to write down a explanation of all of them. And then I repeat until I understand it properly.