I have tried writing things I don’t want to forget, but I didn’t like it much. I have copied short quotes and added sticky notes at important parts, which I liked, but felt possibly not enough. I tried annotating, but it made me feel bad and disrespectful for the book.
Buy a notebook, take a pen and put both to the side of your book. Read from the book first, going paragraph per paragraph. Understand what is being said, and take your time trying to figure it out if it is a hard and complicated part (there's many of those in Kapital). Then, once you have it, try to put it in the simplest of words without making it lose its essence. Finally, write it down.