The most effective letters of recommendation are written by professors or work supervisors who know you well enough to describe your academic, personal, or professional achievements and potential with candor, detail, and objectivity. Letters that compare you to your academic peers are often the most useful. Most schools do not consider general, unreservedly praiseworthy letters helpful.
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.