The first thing to do is choose a subject. Your professor will either give you guidelines (such as a family member) or allow you to choose whomever you please. Of course, it's necessary to have the cooperation of you interviewee. Regardless of how interesting you think the person is, if they don't wish to be interviewed, you'll need to choose someone else. Don't overlook people who you feel don't have a lot of interesting tidbits to offer. Everyone has a story and you might be surprised at all the things you didn't know about a seemingly boring person.
For the best results during the interview process, be sure that your subject is comfortable. While it may be easier for you to work with a video camera or audio recorder, that may really put off your interviewee. Check ahead before bringing any extra recording equipment. At the least, set it up in an unobtrusive location so that your conversation can flow freely. Also, don't be afraid to delve deeper into your subject's life. You will often find the true gems by allowing your subject to jump from topic to topic more naturally. Come with a standard list of questions to get things going, but don't be afraid to deviate.
The next step is to organize the information that you obtained during the interview process. Luckily, the interview essay is ideal for a lot of wiggle room in your outline, structure and format. The interview essay is fairly laid back, so you don't need to over think things. There are, however, several common ways to create your outline. Here are two examples to get you going:
Create a time line, presenting interesting facts and events that follow your subject as she grows up.
You can also present the information separated by topics and similar life experiences.
Now that you have an outline and know how you are presenting your information, it's time to bring it together and write your interview essay. The biggest challenge in writing is often just getting enough momentum to get started. The best advice is to just go for it. You can always come back later and edit for content. However, you will never finish if you don't start. Staring at a blank screen can be intimidating, so don't let it stay that way for long, even if you are unsure about your ideas. It's also a good idea to re-read your paper as you are writing. Stop every few paragraphs and read your words aloud. They should flow logically and be easy to understand. When you use this proofreading technique, you will often pick out things that need fixing much more easily.
Once you have all your ideas on paper, you will need to do the final proofreading and editing. Hopefully, your spell checker picked out the major mistakes. However, you should still pay careful attention to grammar and punctuation rules. Here are a few common mistakes:
Incorrect Possessive Pronouns: its, theirs, yours, hers, etc have NO apostrophe. Its and it's are commonly mixed up. A common way to determine which is correct is to substitute "it is" in your manuscript.
Mixed Up Forms: There/their/they're and your/you're, whose/who's, etc. There denotes a place, their suggests possession, while they're is the contraction for "they are". Another common mistake is to mix up whose and who's. Who's is the contraction for "who is", while "whose" is a possessive form.
Paragraphing: Your paragraph splits should be logical. Each paragraph should deal with the same subject matter and sequence of events. Also, when quoting dialogue, each new speaker requires a new paragraph. Also, you should avoid things like run on sentences when constructing your paragraphing. Do not overuse and, or and but.
Consistent Voice: Your entire interview essay should be written in the same tense. If you start out talking in third person, keep it that way throughout. Also, make sure your tenses are also consistent. Do not jump from past to present in the middle of your paper.
Avoid Overuse: Many writers use the same words over and over. Most common words have an equivalent substitute. Pull up the thesaurus if you need a few more ideas.
Of course, there are dozens of others, but this should give you a good place to start. For other common grammar issues, do a general search on "common grammar errors" to obtain a full list.