How should I approach the big red book?

So I’m going to start prepping with the official ACT book soon but I don’t know where to start. Time isn’t too much of a problem but I still need to work on it. So should I start by first doing untimed tests and making sure I understand it or doing timed tests? Any advice will be helpful.

Timed tests! Timed tests! Timed tests! Then review your mistakes and don’t make them again.