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.
Then, explain how you know the person and why you’re qualified to recommend them. Include specific examples of the person’s qualifications and successes in the body of your letter, then end with a sentence affirming that you recommend the person for the job or position.