I teach GED in a prison to the Steck Vaughn series. After a decade in the classroom, I've probably put four or five hundred guys through the test. Many of the guys who've passed through my class have scored high enough to earn scholarships once they released. Colleges normally have scholarships reserved for GED holders and a high score is how to get them. I love the Science book! It's part of my bread and butter to get high scores on the GED.
Anyone who uses this book the way it's written will pass their science test with a high score. I've always supplemented the book with my own knowledge just to keep class interesting, but mostly...I just used the book the way it's designed. I would suggest working through the book cover to cover and follow the directions. The answers are in the back. Check your work and understand each lesson before moving on. Don't skip the first part of each chapter that teaches an important skill. That's the most important part of the lesson.
It's divided into areas that teach things like "contrast and compare" or "main idea". Learn those skills. Content varies depending on which form you take. The main skills don't. Do study the content, though. It's easier to remember something than it is to learn it for the first time, and some of the content in this book is on the test. Follow all the directions including the extra exercises between the chapters and timing yourself on the ten minute mini-tests. That helps with your timing.
I recommend this book highly. Anyone who uses it the way it's designed will pass their GED test and probably get a very high score.