You make it professional. Concise and only one page long. Consider it an overview of your resume. Highlight your strengths and provide a brief background of your work experience. If your mailing it via snail mail you will need to add the proper address information in letter format. As in your address and the company's address. I've also X'd out some stuff to protect the innocent.
I've attached the one I've used in the past. Try to make each letter to fit the position don't use a generic one. This one was for a freight finance manager so I emphasized my skills in freight and finance. If I was applying for a position heavy on EDI I would go heavy on my EDI skills and light on the finance freight stuff.
Good luck!