First of All, it is very important to understand where you stand at this point of time. Hence, Please take a full length mock on Day-1 to analyse what your verbal scores are like. Once you have a base, It is easier to work towards a target.
Whatever the score, it is always better to start with Sentence correction lessons and get a hang of the grammar rules in place at the same time. There are multiple topics inside Sentence Correction where GMAT tests you such as:
1. Subject Verb Agreement
2. Verb Tenses
3. Modifiers
4. Parallelism
5. Comparisons
6. Pronoun Errors
7. Idioms.
8. Miscellaneous Ones.
Initially, start doing topic wise practice, until you solve about 30 Questions per topic and then move towards assorted topics. Considering you have just 30 days to prepare for GMAT, keep the first 7 days for SC completely and maybe start doing a bit of reading on random topics from multiple sources.
Once you are done with SC, Move towards CR - Which happens to be Critical Reasoning. Here, its very important for you to understand the argument completely. It is not possible for you to go ahead and solve the question unless you know what a premise, conclusion or an assumption is. Also, this is going to help you even with RC, considering many RC questions are based on the assumptions of the arguments. There are multiple topics in CR, just like you have in SC, such as the following ones :
1. Strengthen the Argument
2. Weaken the Argument
3. Identify the Assumption
4. Identify the Conclusion
5. Evaluate the Argument
6. Identify the paradox
7. Bold Phase Questions
8. Miscellaneous Questions
Initially start with the assumptions questions, as they are most tested on GMAT. I would suggest from day 8 onwards, do CR and RC on a daily basis. This will also improve your reading speed. Keeping a check on the speed and accuracy is mandatory as you cant let go of either. GMAT is going to penalize you if you don't finish the exam as-well.
Finally, comes the Reading Comprehension Section. It is very important to have good speed and high comprehension when it comes towards reading a particular passage. In the exam front, you have to be very diligent with your time and ensure you find out which part of these three are your strengths and weaknesses. Ensure RC is never a weakness and spend enough time on this part of the exam on a daily basis.
Also, when it comes to RC, it is always advisable to understand the structure of the passages provided. Once you understand the role of every paragraph, it gets a lot easier for you to answer the questions that follow up.
Also, it is very important to build your stamina from day 1. Considering you have only 30 days to appear for the exam, please make sure you give at-least 5-6 mocks around the same time as your exam slot before the D-Day. Ensure most of your practice sessions also happen around the same time as that of your final exam.
Exhaust all official sources as they are closest to what you will see in the real exam.
All the best.
K S Raksha
GMAT Tutor at Wizako GMAT Classes & Online Courses