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.
Target the recommendation. Is it for an academic post, a job, a volunteer position, a background check, or a personal reference? Write your letter so that it is focused on this purpose. For example, if the letter is part of a job application package, it should focus on the professional qualifications and conduct of the candidate.