The purpose of highlighting and making notes as you read through your textbook is to synthesize the information you read in a way that will help you gain a deeper understanding of the material. You can also reduce your overall reading time by reading the material thoroughly once, selecting the important information, and then focusing your attention on that information in your notes.
Develop your own marking/highlighting and note-taking systems. Here are some suggestions you may find helpful for marking your textbook.
As you read and mark your reading materials, you may want to make notes in the margin. Make these notes brief. Make marginal notes to indicate something of importance, an example, the number of parts/steps, an important term. For long pieces of reading, notes of this kind in the margin can serve as a way to find information in the text.
Now that you have made notes in your text, you can use your highlighting and marginal notes to study. Reread only the information you have highlighted and marked. Try to make sentences that fit together with the marked information. Finally try to recite and write the important information you have marked from memory.
Most importantly, check your understanding of the material. Often times, we read information, think we understand, but don't check ourselves. So, quiz yourself and double-check your answers!