Describe how you know the person. Give some context for your recommendation. Let the reader know how you met them, how you worked together, and your basic qualifications.
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.