Hi Fashil96,
The good news is that 650 is not a bad start. To improve your GMAT score to a higher level, first and foremost, I think studying quant and verbal simultaneously is a good move. For example, if you study for 3 hours on any particular day, study quant for 1.5 hours and verbal for 1.5 hours.
Regarding how to improve your verbal and quant skills, you need to go through GMAT quant and verbal carefully to find your exact weaknesses, fill gaps in your knowledge, and strengthen your skills. The overall process will be to learn all about how to answer question types with which you currently aren't very comfortable, and do dozens of practice questions category by category, basically driving up your score point by point. For example, if you find that you are not strong in answering Number Properties questions, then carefully review the conceptual underpinnings of how to answer Number Properties questions and practice by answering 50 or more questions just from Number Properties: LCM, GCF, units digit patterns, divisibility, remainders, etc. When you are working on learning to answer questions of a particular type, start off taking your time, and then seek to speed up as you get more comfortable answering questions of that type. As you do such practice, do a thorough analysis of each question that you don't get right. If you got a remainder question wrong, ask yourself why. 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 carefully analyzing your mistakes, you will be able to efficiently fix your weaknesses and in turn improve your GMAT quant skills. Number Properties is just one example; follow this process for all quant 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, types that you would rather not see, and types 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.
You can work on verbal in a similar manner. Let’s say you are reviewing Critical Reasoning. Be sure that you practice a large number of Critical Reasoning questions: Strengthen and Weaken the Argument, Resolve the Paradox, find the Conclusion, Must be True, etc. As you go through the questions, do a thorough analysis of each question that you don't get correct. If you missed a Weaken question, ask yourself why. Did you make a careless mistake? Did you not recognize what the question was asking? Did you skip over a key detail in an answer choice? Getting GMAT verbal questions right is a matter of what you know, what you see, and what you do. So, any time that you don't get one right, you can seek to identify what you had to know to get the right answer, what you had to see that you didn't see, and what you could have done differently to arrive at the correct answer.
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.
Regarding resources, you should check out the GMAT Club reviews for
the best quant and
verbal courses, and also read through some
GMAT success stories to see what materials have worked well for other test-takers.
Lastly, you may find it helpful to read the following article:
How to Score a 700+ on the GMAT 
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13 Dec 2019, 02:40
Fasih Ur Rehman_359688633_ESR.pdf [506.85 KiB] 
