To improve your quantitative score, I believe that you should compartmentalize your learning and take the time to master each topic (e.g., percents, ratios, exponents, statistics, etc.). This means that, for each topic, you should:
1) Learn the underlying concepts (rules, attributes, notation, etc.)
2) Learn GMAT-specific strategies related to that topic
3) Practice dozens of questions all related to that one topic.
4) Don't stop working on that topic until you have mastered it

Then, and only then, move on to the next topic.

To help you focus on one topic at a time, you can use BTG's tagging feature. For example, here are all of the questions tagged as statistics questions: https://www.beatthegmat.com/forums/tags/ ... statistics
See the left side of that linked page for more tag options.

While completing questions from the Official Guide (OG), you should you use an error log (aka Improvement Chart). You can find a free downloadable Improvement Chart here: https://www.beatthegmat.com/mba/gmat-error-log. This will help you identify and strengthen your weak areas.

You should also spend a lot of time reviewing the responses from the Experts on this site. They model the steps one should take when tackling math problems.

In addition to learning the core concepts and GMAT-specific strategies, be sure to work on your endurance and test-taking skills (e.g., time management) by taking several practice tests. If you're interested, we have a free GMAT time management video at https://www.gmatprepnow.com/module/gener ... es?id=1244

Finally, since you have about 3 months to prepare, you might consider signing up for Beat The GMAT's free 60-Day Study Guide (https://www.beatthegmat.com/mba/gmat-guide).
Each day, you'll receive an email with a series of learning activities that guide you, step-by-step, from Day 1 to test day. This will ensure that you will cover everything that the GMAT tests.

Here's an outline of all 60 emails: https://www.beatthegmat.com/mba/gmat-guide-outline

Cheers,
Brent