Generally speaking, the most convincing reference letters will be those written by a supervisor. If you have known the person for a long time, be sure to emphasize this. At first glance, they seem like a completely different animal from a regular recommendation letter, but they are essentially just a more concise version.
Annie was a joy to work with because of her amazingly positive attitude and eagerness to stick tightly to deadlines. The recommendation might explain why they have leadership potential or how they have succeeded in past academic or business pursuits.
If there is a submission deadline, be sure to submit it prior to the due date. Include details of the applicants positive personality traits and examples that clearly showcase them.
Ask them for an example or two of jobs they are applying for.
Use the following recommendation letter template as inspiration when drafting your own: To Whom it May Concern: It is my pleasure to strongly recommend [Applicant Name] for [Position With Company or Acceptance to Institution].
Follow the submission guidelines. Your examples in this type of letter will be more broad, and less specific.