Ask yourself questions about your life, then write out the answers in an emotionally engaging way. The first question you should ask yourself is quite simple: Where and when were you born? But within that one dry question hides a host of others: Why were your parents there, at that particular time and place? How did the surroundings of your early childhood affect your life? What impressions of it do you still carry around with you today? Keep writing, exploring the years of your childhood.
Start a new chapter and describe your teenage years. Where did you go to school? What did you dream of doing after graduation? Who were your friends? Your purpose in answering these questions is not so much to recite a dry series of names and dates, but instead to evoke the feeling of being there, in that time and place. Choose the most vivid memories you have and focus on describing them.
Write a new chapter for each major part of your life -- a certain place you lived, a particular job you worked, and so on. As you describe each part of your life, try to engage all of the senses. What did a particular place look like, exactly? What sounds did you hear? What were the scents in the air? What did it feel like to be there, at that moment?
When you finally get to the present day, write a chapter summarizing what you have learned thus far in your life. Focus on the single biggest lesson you carry with you, the thing that could best help someone else who faces a similar life experience. By doing this, you invite your reader inside the story of your life, including them in a way they will never forget.
When you have finished writing the manuscript, set it aside for a while, so that you can gain some perspective on it. Let it rest for a whole month, if you can. It might help to put it in a box and hide it in the closet. During this time, resist the urge to take a peek. By spending time away from the manuscript, you are giving your emotions a chance to settle down, so that you can go back to your story with a fresh perspective.
After a month has passed, take your manuscript back out again, settle down in a comfortable place and read through the entire thing, from beginning to end. If something jumps out at you, perhaps a detail that needs to be explained or a fact that must be checked, make a note in the margins with a colored pen. But don't start revising until you have finished reading the entire manuscript.
Go back through your autobiography, one chapter at a time, and make those changes. When you are finished, give it to a trusted family member or friend. Even if you don't intend to publish your book, you might consider having a copy bound for yourself at a local copy shop or book binder. It will make an heirloom that you can treasure for years.