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.
Get feedback. If you’re not sure of your letter-writing skills, or if your letter will carry a great deal of weight for the candidate’s application, ask for feedback from a trusted colleague who may also know the candidate. If you are putting your reputation on the line for this person, you want to put your best foot forward with this letter.