You ask a really great question. Since I am unsure whether you have already taken the GMAT, I will assume that you are starting your GMAT prep from scratch. So, before doing anything else, you need to familiarize yourself with the GMAT and then take an MBA.com practice exam. Your experience taking that test will give you a good idea of what to expect on the GMAT, and the results will serve as a baseline GMAT score. Once you see how far you are from your score goal, you can more easily predict your preparation time. I also wrote a detailed article about how long to study for the GMAT, which you may find helpful.
After completing your initial practice test, you will need to devise a detailed, thorough, and linear study plan. In other words, follow a study plan that allows you to slowly build mastery of one GMAT topic prior to moving on to the next. Within each topic, begin with the foundations and progress toward more advanced concepts. Following such a plan will allow you to methodically build your quant and verbal skills and ensure that no stone is left unturned.
For example, let’s say you are learning about Number Properties. First, you should develop as much conceptual knowledge about Number Properties as possible. In other words, your goal will be to completely understand properties of factorials, perfect squares, quadratic patterns, LCM, GCF, units digit patterns, divisibility, and remainders, to name a few concepts. After carefully reviewing the conceptual underpinnings of how to answer Number Properties questions, practice by answering 50 or more questions just from Number Properties. When you do dozens of questions of the same type 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 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 around at least 90 percent accuracy in your training. If you get 100 percent of some sets correct, even better. Number Properties is just one example; follow this process for all quant topics.
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.
Follow a similar routine for verbal. For example, let’s say you start by learning about Critical Reasoning. Your first goal is to fully master the individual Critical Reasoning topics: Strengthen the Argument, Weaken the Argument, Resolve the Paradox, etc. As you learn about each Critical Reasoning question type, do focused practice so that you can track your skill in answering each type. If, for example, you get a weakening question wrong, ask yourself why. Did you make a careless mistake? Did you not recognize the specific question type? Were you doing too much analysis in your head? Did you skip over a keyword in an answer choice? 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 practicing Reading Comprehension, you need to develop a reading strategy that is both efficient and thorough. Reading too fast and not understanding what you have read are equally as harmful as reading too slow and using up too much time. When attacking Reading Comprehension passages, you must have one clear goal in mind: to understand the context of what you are reading. However, you must do so efficiently, so you need to avoid getting bogged down in the details of each paragraph and instead focus on understanding the main point of each paragraph. That being said, do not fall into the trap of thinking that you can just read the intro and the conclusion and thereby comprehend the main idea of a paragraph. As you read a paragraph, consider how the context of the paragraph relates to previous paragraphs, so you can continue developing your overall understanding of the passage. Furthermore, as you practice Reading Comprehension, focus on the exact types of questions with which you struggle: Find the Main Idea, Inference, Author’s Tone, etc. As with Critical Reasoning, analyze your incorrect Reading Comprehension answers to better determine why you tend to get a particular question type wrong, and then improve upon your weaknesses. Keep in mind that GMAT Reading Comprehension passages are not meant to be easy to read. So, to better prepare yourself to analyze such passages, read magazines with similar content and style, such as the Economist, Scientific American, and Smithsonian.
Sentence Correction is a bit of a different animal compared to Reading Comprehension and Critical Reasoning. There are three aspects to getting correct answers to GMAT Sentence Correction questions: what you know, such as grammar rules, what you see, such as violations of grammar rules and the logic of sentence structure, and what you do, such as carefully considering each answer choice in the context of the non-underlined portion of the sentence. To drive up your Sentence Correction score, it is likely that you will have to work on all three of those aspects.
Regarding what you know, first and foremost, you MUST know your grammar rules. Let’s be clear, though: GMAT Sentence Correction is not just a test of knowledge of grammar rules. The reason for learning grammar rules is so that you can determine what sentences convey and whether sentences are well-constructed. In fact, in many cases, incorrect answers to Sentence Correction questions are grammatically flawless. Thus, often your task is to use your knowledge of grammar rules to determine which answer choice creates the most logical sentence meaning and structure.
This determination of whether sentences are well-constructed and logical is the second aspect of finding correct answers to Sentence Correction questions, what you see. To develop this skill, you probably have to slow way down. You won’t develop this skill by spending under two minutes per question. For a while, anyway, you have to spend time with each question, maybe even ten or fifteen minutes on one question sometimes, analyzing every answer choice until you see the details that you have to see in order to choose the correct answer. As you go through the answer choices, consider the meaning conveyed by each version of the sentence. Does the meaning make sense? Even if you can tell what the version is SUPPOSED to convey, does the version really convey that meaning? Is there a verb to go with the subject? Do all pronouns clearly refer to nouns? By slowing way down and looking for these details, you learn to see what you have to see in order to clearly understand which answer to a Sentence Correction question is correct.
There is only one correct answer to any Sentence Correction question, there are clear reasons why that choice is correct and the others are not, and those reasons are not that the correct version simply “sounds right.” In fact, the correct version often sounds a little off at first. That correct answers may sound a little off is not surprising. If the correct answer were always the one that sounded right, then most people most of the time would get Sentence Correction questions correct, without really knowing why the wrong answers were wrong and the correct answers were correct. So, you have to go beyond choosing what “sounds right” and learn to clearly see the logical reasons why one choice is better than all of the others.
As for the third aspect of getting Sentence Correction questions correct, what you do, the main thing you have to do is be very careful. You have to make sure that you are truly considering the structures of sentences and the meanings conveyed rather than allowing yourself to be tricked into choosing trap answers that sound right but don’t convey meanings that make sense. You also have to make sure that you put some real energy into finding the correct answers. Finding the correct answer to a Sentence Correction question may take bouncing from choice to choice repeatedly until you start to see the differences between the choices that make all choices wrong except for one. Often, when you first look at the choices, only one or two seem obviously incorrect. It may take time for you to see what you have to see. Getting the right answers takes a certain work ethic. You have to be determined to see the differences and to figure out the precise reasons that one choice is correct.
To improve what you do when you answer Sentence Correction questions, seek to become aware of how you are going about answering them. Are you being careful and looking for logic and details, or are you quickly eliminating choices that sound a little off and then choosing the best of the rest? If you choose an incorrect answer, consider what you did that resulted in your arriving at that answer and what you could do differently in order to arrive at correct answers more consistently. Furthermore, see how many questions you can get correct in a row as you practice. If you break your streak by missing one, consider what you could have done differently to extend your streak.
As with your Critical Reasoning and Reading Comprehension regimens, after learning a particular Sentence Correction topic, engage in focused practice with 30 questions or more that involve that topic. As your Sentence Correction skills improve, you will then want to practice with questions that test you on skills from multiple Sentence Correction topics.
In order to follow the path described above, you may consider using an online self-study course, so take a look at the GMAT Club reviews for the best quant and verbal courses. You also may find it helpful to read this article about how to score a 700+ on the GMAT.
Feel free to reach out with any further questions.