The best way to write a letter of recommendation is to start out with a formal greeting, or using "To whom it may concern" if you don’t know the recipient. Follow the greeting with 2-3 sentences of praise for the person you’re recommending.
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.