Most people start with the main content and strategy chapters, which goes up through the FDP Strategies chapter. The Extras chapter is harder stuff - save that for later (or possibly never!).
Go through a chapter, do some (or all) of the problems at the end of the chapter, then do a couple of the OG problems (the problem lists are at the end of the book). Then go to the next chapter.
When you finish a book, do a problem set of 5 to 8 questions that are mixed up across all of the chapters.
When you finish a second book, do a problem set of 8 to 10 questions that includes problems from the first and second books. When you finish a third book, do a problem set that includes problems from all 3 books. And so on. That'll help you review as you go - and it'll also help you learn how to figure out what each problem is (since the real test will mix everything up too).
Also, I recommend that you read these (in the order given):
https://www.manhattangmat.com/blog/inde ... lly-tests/http://www.manhattangmat.com/blog/index ... -the-gmat/There are links to other articles in the above - follow those.
Have you taken a practice CAT yet? If not, do so sometime soon. I know you don't feel ready - nobody does! The point, though, is to learn something about your strengths and weaknesses, which can help you prioritize your study. It will also let you know how you're doing on overall timing and stamina so that you can tackle those areas sooner rather than later. (You'll understand the importance of these areas after you've read the above articles.)
Use the below to analyze your most recent MGMAT CAT(s):
http://www.manhattangmat.com/blog/index ... ice-tests/Then come back here and tell us the results of your analysis and what you think you should do based on that analysis. We'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)