It's best to get a sense of where you are beforehand, so you know what areas to focus on.
Take a timed test and take the entire test in one sitting. When you're done, evaluate your results and take notes on what sections and types of questions you did well on, and what you will need to work on.
Taking our diagnostic test makes all of this a breeze.
Estimate the total hours of study time you have from now until test day. If you have time, spread your prep out to an hour or two a week. Divide you time so that 60% of your time is spent on English and Math sections and 40% is spent on Reading and Science sections. You can also try to spend more time on the sections you scored the lowest as it offer the most opportunity for score improvement.
When you sit down to begin studying for a section, start by learning about the structure and content of the section. Familiarize yourself with the instructions for the section. Work through practice questions for that section. Take notes on the questions you struggle with and continue to practice similar questions.
Continue to take practice tests to familiarize yourself with the test, identify areas you have trouble in, and work on your test taking strategies to build confidence. When going over practice test results, write down questions you have particular trouble with on flashcards. One approach to practice test is to take a practice tests for the section you just reviewed. After you have reviewed all four sections of the test, you can take a full practice test.
You should be done studying for the test in the final week before your test. Ease off the studying time a bit, but continue to review your notes, flashcards, etc. You will also want to take and review a final practice test.