Given the 30-minute time limit, you might be tempted to start writing your essay immediately. Nevertheless, it pays to take some time to consider the prompt carefully. The official Educational Testing Service, or ETS, site for the GRE states explicitly that the student must “consider the logical soundness of the argument rather than agree or disagree with the position it presents.” Thus, you need to study the argument carefully, examining it for any logical flaws. As you read the prompt, identify the main argument being made and note how the argument is supported.
After you have examined the prompt, you should begin constructing a rough outline. Even if you don't usually outline before writing essays, you should for the GRE writing tasks because you won't have time to make major revisions. Spend about five to 10 minutes planning the essay before beginning to write. Each of your planned paragraphs should focus on analyzing the argument given in the prompt, usually by exploring a flaw in the argument's logic. ETS's study guide for the argument task directs test-takers to "examine the argument’s unstated assumptions and discuss what the implications are if the assumptions prove unwarranted."
Once you have created an outline for your essay, it is time to begin writing. There is no required length for the essay, but the examples of high-scoring essays provided by ETS are usually considerably longer than lower-scoring essays. Thus, it is best to write quickly, developing each of your points as completely as possible with examples and other relevant details. If you get stuck on one paragraph, move on to the next and come back later. At this point, you should not waste time revising and correcting your writing -- just get your arguments down as completely as possible. Watch your time carefully, though, leaving about five to 10 minutes to revise your work at the end of the session.
During the last five to 10 minutes, take time to refine your work. Read over the essay and check that it reads smoothly. Also, be sure that you have stayed focused on analyzing the argument, without straying into personal opinion or off-topic details. If needed, add transitional phrases between ideas and remove any redundancies. Make sure your sentences are as clear and concise as possible and correct any errors in grammar, punctuation, spelling or style.