Your first step is planning. You will want to make sure that you are giving yourself plenty of time before the actual test to study and take practice tests. Three months should be sufficient and anything more than six months is probably a waste of time.
Next you should select a good study guide. You do not have to pay for an expensive test prep course. Reading a good study guide and taking old GREs should be sufficient for your preparation work. If you look on Amazon you will find reviews for the various study guides that are available and you can make an informed decision as to which one is best for you.
Your next step is to read your study guide and online resources so that you can learn the exact mechanics of the test and some strategies for how you should complete the sections.
Now that you have a good working knowledge of the test, it is time to practice like crazy. You should take practice tests and keep track of how your score improves and which sections you need to work on. A good place to find practice tests is in your university library if you are still in school.
When you take your practice tests you should try to simulate the testing environment as much as possible. Go to a quiet place where you know you will not be disturbed and try to take the test all in one sitting with a few breaks, just like the real exam.
If English is your second language, some of the verbal and written tasks may carry extra difficulty for you. A good way to improve your advanced English skills for these sections of the test is to acquire some technical science magazines or magazines like the New Yorker that use more advanced language. Try reading these magazines and writing brief reports on some of the articles that interest you.
If you have followed the steps, you should be able to perform to the best of your ability when you take the actual test.