Familiarize yourself with the position. Get a copy of the job description, if you can, and talk to the person you will recommend. If you know the intended recipient of the letter, ask them about the position as well. The more you know about the position, the better you will be able to tailor your letter to suit the needs of both parties.
Use a business closing and sign your name. Above all, be professional. If the you’re sending a physical letter, print it out and sign it by hand. Otherwise, just typing your name will do.