If you have an option of preparing through offline and online it will be great for your preparation.
You can follow the below ways to prepare for GRE:
1. Start your work by creating a realistic schedule of daily work. Because of the reason that the daily part is important, realistic probably translates to 15-20 minutes a day.
2. Use the official materials. There is only 1 published book for the GRE Examination.
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3. Materials for study purpose include practice problems and practice tests. Do the practice problems first, then start with the regular practice testing as the actual test date approaches. There are two official GRE practice tests provided that are free to download. Because the GRE tests are adaptive, they can be taken at any multiple times. You can get away with approximately around three sittings per test without significant overlap, so that it allows you six practice tests.These should be taken at least once a week for the six weeks leading up to the test, in an environment which simulates the actual GRE testing environment as closely as possible.
4. Find someone with whom you are able to review the questions you get wrong. The person need not be a professional if that’s not feasible for you, but it should be someone who’s at least good at the relevant content, or who has scored well on the test before, or both.
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5. Solve more practice problems for the content areas that seem to be covering the highest frequency of incorrect answers on your practice tests.
6. Even though there is no official vocabulary list, GRE prep requires studying vocabulary. Antonyms and analogies lean more heavily on mere vocabulary than the passage or vocabulary in context questions
7. Finally, just to sum up — consistency, planned execution of preparation is the key. On tests like GRE, cramming is basically a giant crap shoot.
All the Best!!