Choose from one of the essay prompts given to you by the university. Typically, universities will offer at least three essay choices. Ensure that you select the prompt that will most likely encapsulate the type of person you are and allow leeway for indicating some of your academic goals at the university.
Spend approximately one to three hours brainstorming and writing ideas down for how best to structure your essay. The more you write down, the more you can sift through and use the best ones to fine tune your approach.
Write your essay using the concepts you've derived from your notes, making sure to cover as much ground as possible in the word limit you're given and ensuring that you completely answer all parts of the question.
Read your essay, checking for any technical errors and then for any major changes in paragraph configuration and sentence modification. Once you have reread the essay yourself, give it to someone else (ideally, a teacher or guidance counselor with an invested interest in your future) to get a second opinion on any other changes you should make before submission.