Brainstorm a list of important events that have happened to you. Write them down in a journal. An "event" doesn't need to be a literal incident; an event can be the year before you moved to college, the month you lived in Thailand or a long recollection of everyone's reactions on the day of a family member's funeral.
Look at the events you've listed and choose one that you want to write about. Make sure you have a lot to say about this topic. From your one topic, break your book down into chapters or small sections--often these small sections are called vignettes. For example, if you want to write about your experience in Thailand, you may want to write one chapter on why you decided to go, another on what happened while you were there and a last about how you've changed as a person after visiting the country. Before writing your chapters, create a web or outline illustrating what you'll write in each section.
Begin writing your memoir by looking at your web. Tackle the first section or chapter, then tackle each chapter one at a time. Be honest and detailed. Set up the situation so the reader can really see where you are coming from and how you lived. According to Readers Digest, even if your story isn't very dramatic or wild, you should impart on your readers what you've learned from life and what you got out of your experiences. That is what makes a memoir unique.
Edit the memoir thoroughly. Proofread carefully to find any blatant errors. Be as honest as possible when writing, and make sure any quotes or dialogue pieces reflect the truth as much as possible. Change names if your relatives or friends feel uncomfortable, according to memoirist Linda Joy Meyers.