Decide just how brief you want the story of your life to be. It's easy to feel overwhelmed at the task of chronicling your life, but committing to a minimum and maximum number of pages will help you pace yourself in order to avoid burnout.
Create a detailed outline of your life. This will serve as an outline for your story as well. For the sake of brevity, focus first on pivotal events. Then outline other notable happenings with one- to two-sentence notations. When complete, the outline should be no longer than one or two pages total. This might vary depending on age and personal experience.
Consult older family members, friends and important people in your life who can provide further background information on events that took place when you were young. These talks will not only help you flesh out your story, but will jog your memory too. Keep in mind that the purpose of their perspectives is to aid you in solidifying your own.
Reference photo albums, letters, awards, souvenirs and other memorabilia that can stimulate your memories and thus enrich your written account of them. Make brief notes for each one describing how these things made you feel and what they meant to you at key points in your life.
Write the story of your life. Use descriptive details and language when describing the biggest events (use your own perception to determine these). Try to avoid rambling when writing about less significant periods. Following your outline will help you chronologically order the story as well as organize your writing. Take breaks from writing if you need to, but don't feel obligated to edit your work to fit anyone's expectations except your own.