Break down the letter into short, but specific, paragraphs. Don’t ramble, but do flesh out your key points. Try to keep it at one page, covering the various aspects of their recommendation in brief, specific detail.
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.