Be specific about the candidate’s qualifications and successes. Describe what the person has done using specific instances and examples, rather than generalities. Whenever possible, give evidence or stories to back yourself up.
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.