The best way to approach someone you want to interview is by personal contact, rather than by letter, and often the initial contact will be by telephone. This gives you an opportunity to introduce yourself, explain your project and outline the sort of topics you might cover in your conversation. The person you have approached may be uncertain: they might say they have nothing interesting to say. So you sometimes have to do a bit of persuading. The key is to talk in terms of “a chat about the past” or a “story of your life” rather than an “interview” which can sound forbidding!
When you speak to them get some background information and decide where the interview should take the place. The person’s own home is by far the best as they will be much more relaxed. A one-to-one interview is best. Privacy encourages an atmosphere of trust and honesty. A third person present, even a close partner, can inhibit and influence free discussion.