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.
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.