Hello there,
If you are unable to understand a chapter, divide it in parts. Try to complete one topic at a time, and then make your own notes. Explain the points in your own words. When you are confident that you have understood that portion, you can continue the chapter. There are many videos available in YouTube which explain these topics nicely. You can take help from them. If possible, draw schematic diagrams of the events happening. It is easier to understand and remember schematic diagrams than remembering a whole paragraph. Don't rush to complete the entire chapter in a day, take your time.
Hope this answer helps you.
Good luck!!