Welcome to the forum!

BTG is one of the best, if not the best, forums for the GMAT in my honest opinion.

Anyways, I'm not sure who gave you the advice not to study out of the official guides, but that's TERRIBLE advice. The exams that are found on mba.com doesn't give you questions out of the guides. I can understand "saving" the 2 exams the GMAC gives you(because you can get repeat questions), but not studying out of the OG guides would be crazy. The OG guides are what you should be basing the core of your studies on, as the questions in the guides are the most GMAT like questions you can get. The MGMAT guides that you're interested in assigns all of their practice problems (not the "in action" problems at the end of each chapter) out of the official guides.

As for which supplemental books to use along with the OG, I personally went with MGMAT. The guides are great and in depth, and really helped to up my score. The best advice I can give when you're going through the guides is to trust the techniques they give you! Trust me, I had to learn the hard way as I'm pretty stubborn. When they tell you to diagram critical reasoning, you should most likely diagram the critical reasoning. The techniques they teach are definitely essential.

I also recommend the Powerscore CR Bible. The book definitely helps with understanding how the GMAT thinks to critical reasoning.

Thursdays with Ron also give little random nuggets of gold, I highly recommend them.

https://www.manhattangmat.com/thursdays-with-ron.cfm

The OG12 itself takes a long time to go through if you're properly doing the problems and properly reviewing them, add the MGMAT books and the supplements that come with the books, you'll pretty much have no free time for the next few months lol!

Good luck with your studies, everyone starts this journey at a different point; some take 1 month to prepare and get the score they want and some take 2 years to get the score they want. Don't be easily discouraged!

I have to agree with Chendawg. The OG is the starting off point. You'll need to master it and the Powerpreps. Beat the GMAT has a great little review of books here:

I personally recommend the MGMAT Number Properties and the Powerscore CR. https://www.beatthegmat.com/mba/books

I think the best study-plan utilizes the OG material, 2-3 supplemental books, and then the best online resources like Grockit, Beat the GMAT, and the Powerpreps.

Good luck!