Active learners do more than just read and underline the key ideas in the textbook.
Active learners write about what they have learned. No matter what type of notes you take informal outlines, summary paragraphs, lists of key ideas you should aim to put the material into your own words. If you are not able to restate the ideas in your own words, then you do not yet understand the material and will need to go back to your reading to figure it out. If you can restate the ideas accurately, then you are on your way to learning the material. So find a quiet spot and start talking to yourself.