Professional football teams hold an open draft where you can show off your skills. If you have outstanding stats that you feel have gone unnoticed, you can detail them in a letter, but showing up to prove your mettle is key.
You start out your letter as if you were writing to anyone in a letter. Use to whom it may concern, or to whichever football team your interested in. Write the body, and then end it with a sincerely and your name.