Familiarize yourself with the position. Get a copy of the job description, if you can, and talk to the person you will recommend. If you know the intended recipient of the letter, ask them about the position as well. The more you know about the position, the better you will be able to tailor your letter to suit the needs of both parties.
You are required to have three letters of recommendation submitted on your behalf. The letters should be submitted by instructors in a discipline related to your proposed field of study who are in a position to analyze your abilities and academic promise. Letters may be submitted online or on paper via regular mail; online submissions are strongly preferred.