Become familiar with the structure of the essay. On most timed essays, you won't know the topic until you take the test, but you will know how long you have, what the essay will be graded on, and perhaps what types of prompts are usually given. Look at examples of timed essays from previous tests, if any are available.
Budget your time. Figure out how long you should leave for planning your essay and how much time you should save for writing it. Usually, planning should take no more than a third of your time.
Come up with an idea for your thesis statement. It should be something simple and straightforward enough that you can prove it in the time allowed. Write it down on a piece of scrap paper.
Write down 3 supporting points for your thesis. Come up with at least 2 pieces of evidence for each point. Don't worry about getting the perfect phrasing, or even using complete sentences. Just jot down enough information to outline the basic structure of the essay.
Write the thesis statement down at the top of the page. Your thesis should be 1 to 2 sentences, should make a point and should introduce 3 main arguments in its favor. Write on every other line to give yourself room to revise.
Write your first main point, rephrasing it slightly. Support it with at least 2 pieces of evidence. Your evidence should be concrete facts---quotes from the text, if possible, or at least arguments directly referring to whatever you are writing about. Your opinions generally do not count as evidence---you need to bring in something more substantial to back them up.
Write your second and third points. In general, each point should take up 1 paragraph, 2 at the most.
See how much time you have left. If you have 15 minutes or so left, you may have time to rewrite the essay. If you have less, you will only have time for minor editing for grammar and punctuation.
Go back to any sections of the essay that seemed awkward or incomplete, and rewrite them. Cross out the section and rewrite it on the blank line directly underneath. If you only have a few minutes left, skip this step.
Quickly read through the essay, looking for mechanical problems. Correct any errors in grammar, punctuation, spelling or word usage.