Introduce yourself. Begin the letter by briefly introducing yourself and giving a bit of information about your background. This is not a chance to write your autobiography, but rather a chance to explain where you are coming from and where you plan on going. This can be done in a short paragraph of three to five lines.
State your interest in the program and explain how you fit in. Immediately make it clear why you are interested in the program and why you would make a good addition to their graduate student roster. Be specific by noting courses, professors or even other students relevant to your interests and your plans. Note any relevant skills or awards that will set you apart as a distinct candidate. This can be done in two paragraphs.
Explain your Ph.D. plan and career goals. In a short paragraph, explain what you hope to take away from the program. State your anticipated timeline and what you hope to do with your doctorate -- for instance, become a professor or a researcher.
Edit your letter. Your letter should run to about a standard page, single spaced. Edit the letter carefully for style, word choice and grammar. Sloppy letters will draw doubt and suspicion, so edit closely. If possible, have a reader look for any mistakes.