Hi Sarrahsaadi18,
Here are some great resources that I used which will cover everything that you need to know for the exam:
Official Guide GMAT Review (12th edition or later)
Official Guide GMAT Quantitative
Official Guide GMAT Verbal
Total GMAT Math by Jeff Sackmann
Manhattan GMAT – Sentence Correction
Veritas Prep – online question bank and seven practice exams
First, you have to familiarize yourself with the content of the exam. Some sections (specifically Sentence Correction) are very rules-based and require you to apply specific rules. Also, in Quant, there are many specific concepts that show up over and over again. It really helps to know these like the back of your hand. If I were you, I would start by going through these materials and mastering all of the theory that is on the GMAT.
Next, I would get used to doing practice problems. Give yourself a set of 30 problems and commit yourself to finishing it within 60 minutes. Get used to the timing and rhythm of the GMAT.
Practice problems should make up the bulk of your study time once you have mastered the theory of the exam. At least once a week, you should be doing a full practice exam to measure your progress. Do all the sections - AWA, IR, Quant and Verbal - so that you get used to the endurance round that is the GMAT and you can prepare yourself not to be mentally exhausted by the time you reach Verbal. The Veritas Prep practice exams are very good for this because they will tell you how long you took to answer each question relative to the average test-taker and what percentage of people get that question correct - very useful information to know in terms of focusing your study habits. The tool will also tell you what areas you most need to work on which will also help you focus your study habits.
After each practice exam, note the questions that you got wrong and practice those concepts. In fact, I would dedicate the study day after a practice exam solely to the questions that you got wrong on that exam.
In terms of a study schedule, I would say practice 10-12 hours per week for 12 weeks. This will easily allow you to clear 125 hours of study time, which should be enough to improve your score by 100 points. Do 2 hours per day 4 days per week and then one practice exam each week on the weekend (which will take you 3.5-4 hours).
I realize that you mentioned you only had 20 days until your exam so I would actually suggest delaying your exam and giving yourself 3 months to study. I realize that you may be coming up on school application deadlines, but this will give you a better opportunity to achieve a higher score and get into the school you want, even if you have to delay your studies by a year.
Best of luck!