Interview the subject of the life story, if you are writing a biography. Get them to tell you about their birth, childhood, education, work, love life, any important historical events they lived through and any major themes or important happenings in their life. It is better not to have fixed questions, rather think about the topics you want to cover and then tailor the questions according to where the interview is heading.
Talk to your parents or relatives to get details about your early years or contextual family history, if you're writing an autobiographical life story.
Use photos, personal items, diaries, newspaper clippings and other mementos to recall memories. You can do this if you're creating your own life story, or you can ask your subject to do so, if you're writing a biography.
Organize the information you've gathered into chronological order. Start at birth and continue on to the present day.
Write down the life story. The best way to do this is on a computer, using a word processing program. You could hand write it, or use a typewriter, but it's slower and harder to correct mistakes.