If you're focusing on a specific time period in their life, however, then simply focus on the things they tell you about that time, and write about the pivotal parts of their life in length.
You then need to look at the list of events that were written down, and then decide which ones are going to be discussed in the biography. You need to make notes about the details in each of these events and the feelings that these events created. Get the person to be completely honest about these things, so that you can really capture what happened at that time in the biography.
Then you can start writing the biography. Write the first chapter of the biography by talking about the very first thing that is written down about the life of the individual. You need to set the scene and be detailed, talking about the people involved in this part of the life.
Continue writing about all the events and of course being completely factual all the time. Once you've written it, go back and ensure that the book reads clearly. Edit the grammar and sentence structure, finally, so that the book can be sent off to publishers for consideration. They will get back to you if they feel that the biography is good enough to be published.