Find essay topics which you can use for practice. The Association of American Medical Colleges offers a list of essay questions that shows you the types of topics you can expect on the MCAT. A link to the site is provided in the References section below. You can also find practice essay questions in MCAT study books.
Brainstorm for about 10 minutes on how you'll write your practice essay. Make sure that your ideas address what the MCAT wants you to address. For each writing sample, the MCAT requests that you first explain what the topic means to you, including an example that supports the sentence. You should then provide an example of when the opposite is true, then discuss ideas that resolve the apparent conflict between your "for" and "against" arguments. Keep these ideas in mind while brainstorming, and think of specific examples that you can include in your essay.
Write your practice essay, keeping in mind the remaining time. Total time for the exam is 30 minutes, so if you take 10 minutes to brainstorm, limit your essay to less than 20 minutes. Make sure you practice this on a word processor, because this is how it will be administered on your MCAT exam day.
Review your essay in your remaining time. Make sure that you have logical connecting words between sentences and paragraphs. You should also check for spelling and grammar mistakes.
Repeat the above steps with a new prompt. Continue to practice writing until you feel comfortable about your brainstorming time and final essay.