Do whatever sample questions you find on the GRE (or is it ETS?) website. Download a program called GRE POWERPREP for free from that site. Read what it tells you about how to take the test, the format and timing of the test, etc. It also gives you some tips on how to solve the types of problems appearing on the GRE, so read this if you want. In addition, it has I think 12 sample answers to the essay questions, ranging from very bad to very good, along with graders' comments, so you can see what to do and what to avoid in order to get a very good mark in these areas. The pool of topics used by the people who make the test is available on the GRE website. Browse through this list, and think briefly about how you might answer them, keeping in mind what you learned from looking at the sample answers from POWERPREP.
This program also offers sample problems, and there are a lot of them. Do a fair number of them. Then do one of the sample tests offered by the program. These tests are formatted and timed just like the real thing. After the test, see your results and decided if you like them. If so, maybe do a couple more of the sample problems, and then do the second sample test. If not, then do a lot more of the remaining sample problems, then do the second sample test. If you like your marks on this second sample test, you're probably sufficiently prepared. All the above can be done easily (i.e. at a leisurely, unstressful pace) within a week. If you still don't like your marks, there are of course other resources to help you prepare, but I couldn't tell you anything about them.