I usually use a pencil to circle the ones that are impressive and unclear, but not too heavy.

Teachers use their favorite pens when giving lectures (but it is best to use one color throughout the book).

When waiting for a major exam, it is most brightly marked with a red pen, which is easy to review. (You can also use a red pen when you talk about the key points in class)

Those key sentences and formulas can be drawn with favorite thick color strokes.

This is my method, in fact, I think it is quite good ~ ~ I hope it will help you! !