For starters, go the person and talk to them about how you will go about writing the book. You should discuss every aspect of their lives and which events you are planning to cover. At times, a few people would just talk about a chunk of someone’s life, while others would cover the whole life. It totally depends upon the writer how he/she wants to go about the whole thing and the character who you are covering should accept everything you are planning to write on.
You should use the correct tone and style of writing for the memoir. The whole style should cover the personality of the person perfectly. There is a high possibility that you might make it sound like you, but that’s not how it should be. The overall tone of the memoir should be like the personality you are trying to write on.
Once you have spoken to the person, make sure that you talk about all the aspects regarding his/her life. Take a video camera along just in case you think you might forget something. Ask as many questions as you want and make sure that you have understood the other person thoroughly.
Make an outline for the memoir and cover events in such a way that the person you are writing on enjoys it himself/herself. Always ask the person how they would want their memoir to be written.
After making an outline, draft the memoir and use it as your guideline. You can always show this draft to the person you are writing on and he/she can correct you in case you have taken a wrong turn. After every chapter, repeat the process of drafting things down and sharing it with that person.