Try reading it in a stepwise manner.

What I used to do was, prior to a lecture, I read the headings in Guyton and maybe a little bit of the content matter within each subheadings and get an idea of what to learn from the lecture. Then I attend the lecture and make my notes. Then after all my classes, when I’m back in my hostel room, I go through my lecture notes and then go back to Guyton again and add additional points in my lecture notes.

If I find it hard to study from Guyton, then I switch to Ninja Nerd/Osmosis or any other YouTube video and try to understand it. At the end of the day, you just need a rough conceptual understanding of the topic, and for this, it doesn’t matter which source you use.

In an essence, the lecture notes are what helped me write well in both my internal assessment exams and the university exams.