Inform yourself about the person you are recommending. Spend some time talking to them about their goals and the position they are applying for. Gather together their current resume, any memos or notes you may have on them, and any other information that will help you as you write the letter. The best recommendations are thorough and specific, and having all the information at hand will make the process much easier.
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.