Contact the person you want to interview, the interviewee, via email or phone. Explain to him the purpose of your essay, what information you need and explain that the interview may be published.
Receive written permission to publish the name of the interviewee and the information she gives you. Organize the written permission as a contract that explains any conditions you or the interviewee must uphold. For example, include that the interview can only be published in a single essay and not future works, if this is so. Have both parties sign and date. Keep a copy and give a copy to the interviewee.
Prepare a list of questions beforehand and type them on a piece of paper. Write one question per page to leave yourself room for notes. Write open-ended questions instead of “yes-no” types.
Dress in clothing more formal than you expect the interviewee to wear. Formal clothing shows the interviewee that you take your essay and the interview seriously. He may open up to you more.
Arrive to the interview five minutes early. Earlier or later than this and you may annoy your interviewee. If you arrive at the location more than five minutes early, stay in your car. If you plan to be later than five minutes, call ahead and ask if the interviewee would rather reschedule.
Introduce yourself and spend the first five minutes in small-talk. Get to know each other. Transition to the interview by explaining your essay and the information you seek. The interviewee should already know this, but it helps set the stage.
Pursue your questions one by one. Deviate when necessary. One answer may produce another question. Ask the new question before returning to the questions you've already written down.
End on time. If you require more information, ask if you can schedule another interview. If the interviewee refuses, don't push the issue.
Leave the location, even if it's a third-party site such as a coffee shop. This creates a complete break from the interview, especially if the interviewee plans to stay.