For textbooks, I expect to read the material 3 times.
First time I skim through and focus on the headings, illustrations and their captions. That gives me an idea what the chapter is about.
Then, when I'm feeling pretty alert, I read the second time more carefully, take notes, highlight etc. It may take more than one sitting to do this, or if I have a long study session I will take short breaks to walk, eat, get some fresh air. After this, I do any assignments from that material.
Finally, before the test, I skim through the book again, mostly focusing on my notes and highlights.
Sometimes I don't get it at all, in the first reading. But it provides a framework for when I read through the second time. Often, by the third reading, I'm saying to myself, yeah yeah, I know this, and I'm ready for the exam.
Stuff I study before going to bed seems to stick in my memory better.
It helps to start reading as soon as possible to give the material time to process between readings.
I also used to record the teacher's lectures (ask first) and play them back in my car while driving. One teacher loved when I quoted from his lectures in my essay questions. He wrote "Brilliant!" next to an idea that was lifted straight from his own lecture.