How do I start preparing for the Quantitative section of GRE? I am really bad in Quantitative aptitude, even my basics are not good (I am not in touch with Quant practice from the time I left school).

If you have enough time, at least 6 months, then start with the Manhattan. Manhattan has the same level of questions of Quant as it is in the GRE. Start solving the book. There are answers along with the solutions after every chapter. Keep practicing. You will score good.

You should follow what Sandeep has written. Manhattan is the best if you are starting from scratch. Now if you need a comprehensive guide to tackle the GRE then here's my guide. Hope it helps:

Hi There, I work as admissions consultant with Manya Abroad - The Princeton Review and am writing the answer based on my experience. To improve on your quant I would advise you to study one topic at a time and solve all the questions related to it in a one go. Keep preparing notes of the formulas and numbers. Prepare all the topics in this manner.Once completed start solving only the quant section from the GRE practice tests.Once you are able to score well, start solving the section setting time limits and then eventually start attempting the full length practice tests. Practicing this way will definitely pull up your quant score. Hope this helps. All the best