Select a topic that will be interesting enough to warrant a book, at least if you want to get it published. While this does not have to be something major, like the Normandy Invasion, it should have some genuine emotional weight and drama.
Conduct research to gather any information you might need. This might sound strange for an autobiography, but since memory lets us down sometimes, it is best to double-check dates, places, names and other facts.
Create an opening that catches the attention of the reader. At the same time, you will want to supply some basic background information about yourself so the reader sees the events you are describing as not occurring in a vacuum.
Lay out the events that occur in a simple chronological order. Retrospective flashbacks work sometimes in fiction, but they don't work that well in autobiographies.
Write a closing that summarizes your feelings and the effect the event had on you, but do not summarize the book itself, since the reader just read all that.