Understand the memoir genre. In a memoir, you are the main character of your own life story. Many memoirists use the facts of their life story to create an engaging tale for the reader. Because you are relying on your own memories as source material for the story, you may end up describing things differently than others might remember it. The key is to write down things as you remember it, in the most honest way possible. Keep in mind memoirs are different from autobiographies in that memoirs should only cover certain key aspects of your life, not your life from birth until the present day.
Develop a central theme, main idea or thesis that clearly explains the purpose behind your autobiography. For example, your theme might be about how you overcame obstacles to become a stronger person, or it might explain why a particular passion -- such as mountain climbing or rescuing animals -- is so important to you. Back your theme with details and background information about your life experiences. For example, you might explain that your dad is in the military, so you've had to learn how to adapt quickly to new schools and unfamiliar environments. Or, you might discuss what it was like growing up in a poor, single-parent home, but how your family was always willing to take in injured or stray pets.