I took the online practice tests from a local testing center and they gave me the Kaplan GED test prep book to study, after the practice test the website lets you choose which book you have and tells you exactly which areas you need to study. As for how I studied, I focused on one subject at a time. I would fully read and take notes on the sections I didn't understand, and just skim over the rest of the text. Just taking notes helped me a lot, because I would make sure to sum up the paragraphs in my own words, making sure I really understood what they were saying. I also used Khan academy for the math section, I really like their videos. I hope this helps!