Hi Pedro,
So, 660 is a great starting point. With some smart, dedicated prep, you probably can increase your score; however, it may take significant time to reach at 750+.
To increase your quant and verbal scores to a more advanced level, you need to study each topic "with a fine-toothed comb" to determine your exact weaknesses.
For example, if you are reviewing Number Properties, be sure that you practice 50 or more questions just from Number Properties: LCM, GCF, units digit patterns, divisibility, remainders, etc. Once complete, do a thorough analysis of each incorrect question. If you got a remainder question wrong, ask yourself why you got it wrong. Did you make a careless mistake? Did you not properly apply the remainder formula? Was there a concept you did not understand in the question? By properly analyzing your mistakes, you will be able to more efficiently fix your weaknesses and in turn improve your GMAT quant knowledge. Number Properties is just one example; follow this process for all quant and verbal topics.
Each time you strengthen your understanding of a topic and your skill in answering questions of a particular type, you increase your odds of hitting your score goal. You know that there are types of questions that you are happy to see and types that you would rather not see, and questions that you take a long time to answer correctly. Learn to more effectively answer the types of questions that you would rather not see, and make them into your favorite types. Learn to correctly answer in two minutes or less questions that you currently take five minutes to answer. By finding, say, a dozen weaker quant areas and turning them into strong areas, you will make great progress toward hitting your quant score goal. If a dozen areas turn out not to be enough, strengthen some more areas.
In the case of verbal, let’s say you are reviewing Critical Reasoning. Be sure that you practice a large number questions just from that topic: strengthen and weaken the argument, resolve the paradox, find the conclusion, must be true, etc. Once complete, do a thorough analysis of each incorrect question. If you got a weakening question wrong, ask yourself why you got it wrong. Did you make a careless mistake? Did you not recognize the specific CR question type? Were you doing too much analysis in your head? Did you skip over a keyword in an answer choice? Again, you must thoroughly analyze your mistakes and seek to turn weaknesses into strengths by focusing on the question types you dread seeing and the questions you take a long time to answer correctly.
When you do dozens of the same type of question one after the other, you learn just what it takes to get questions of that type correct consistently. If you aren't getting close to 90 percent of the questions of a certain type correct, go back and seek to better understand how that type of question works, and then do more questions of that type until you get to at least around 90 percent accuracy in your training. If you get 100 percent of some sets correct, even better.
So, work on accuracy and generally finding correct answers, work on specific weaker areas one by one to make them strong areas, and when you take a practice GMAT or the real thing, take all the time per question available to do your absolute best to get right answers consistently.
The GMAT is essentially a game of seeing how many right answers you can get in the time allotted. Approach the test with that conception in mind, and focus intently on the question in front of you with one goal in mind: getting a CORRECT answer.
I realize you have some prep materials, so for now feel free to give those a shot. The important thing is to objectively evaluate your progress so that, in the event that the prep books aren’t meeting your needs, you don’t end up overinvesting time in a resource that isn’t working for you. Thus, if after 4 to 6 weeks you find that you are not making the improvement you need, you should perhaps think about changing up your study routine. Certainly that change would not have to be anything drastic, but you may consider using a self-study course.
If you do find that you need a self-study course, take a look at the GMAT Club reviews for the
best quant and
verbal courses.
You also may find my article helpful regarding
how to score a 700+ on the GMAT.
Feel free to reach out with any further questions.
Good luck!