Talk with the person whose memoirs you will be writing to determine the scope of the book. Some memoirs, such as Winston Churchill's "A Roving Commission," confine themselves only to a particular period of a person's life, while others, such as Anthony Burgess' two volume autobiography, attempt to cover an entire life.
Find the right tone and style for the memoir. Both John McCain and Sarah Palin worked with ghost writers on their memoirs, but the prose in "Faith of My Fathers" does not resemble that of "Going Rogue." The style you use to write someone's memoirs should reflect that person's personality and background.
Interview the person thoroughly. Use an audio recorder or video camera to record the contents of your interview. Ask as many questions as possible. Transcribe your recording and read it over several times.
Construct an outline of the memoir. Your book can take a number of shapes. While most memoirs proceed in a linear, chronological fashion, others eschew this traditional approach. Martin Amis, for instance, organizes his memoir "Experience" around topics of interest. Consult with the person whose memoirs you are writing before you consider an approach like this.
Write a first draft of the memoir using your outline as a guide. Show this first draft to the subject of the memoir and ask him to read it. Discuss the changes he requests and keep in mind any suggestions he gives you when you write your second draft. Repeat this process of drafting, sharing and revising until you have a memoir that satisfies its subject.