I don't like to underline the textbook, instead of that what I do is preparing my own notes based on syllabus and previous year question papers with the help of textbooks and bareacts along with caselaws, moreover you should not wholly depend on Old QPs you should also cover the syllabus part also for better conceptual understanding. In case if you don't understand the concepts refer some online blogs and YouTube videos
Then focus on topics taught in class, or marked as important by professor.
If time remains, cover the rest of the syllabus.
Do you underline as you read the text and read through whole section and then jot down points in a separate notebook ?
What is your preferred way and what would you suggest ?