Just write it as a traditional essay, and go right in. No title, just paragraphs. Think of it like your college essays, not a job application cover letter.
You can find examples with a Google search. Don't bother making it fancy, just keep it straightforward. Make sure it is appropriate to submit to all schools, not tailored to any specific school.
If you have not submitted yet, are you sure you want to now? If you are not a perfect applicant, I would rather consider this as a time to get a head start on your application for next cycle, and be ready to hit the submit button early next June.