Identify your readership. If it's family and friends, the language can be less formal and follow a chronological format that starts with childhood and wraps up in the present. Hobbies, travels and memories can be often interspersed. Autobiographies written for business will focus on education, work experience and professional achievements and place only a small amount of personal information at the end, such as spouse, children and city of residence.
List the biggest events and influences of your life. For informal autobiographies, this will include your family, how you were raised, places you lived, schools you attended, jobs you've held, travels and leisure activities. For a business autobiography, identify your major course of study, academic credentials, specialized training, career highlights, awards and leadership/panel/committee expertise.
Create an outline that equitably distributes your content into three paragraphs of about 100 words each. For instance, parse your informal memoir into categories of childhood, work life and fun. You can also divide your current age by three to determine the chronological time frame each paragraph will cover. For a business autobiography, organize paragraphs by present, past and future or spotlight the respective accomplishments of different fields of endeavor, such as acting, writing and directing.
Hook your readers with a catchy opening sentence. Example for an informal essay: "There's no question that being born in an elevator was an appropriate introduction to life's ups and downs." Example for a formal essay: "Since becoming the chief troubleshooter for XYZ nine years ago, I've logged enough air miles to have made a dozen trips to the moon and back."
Write your first draft straight through without paying attention to word count until you're done. Show it to someone whose opinion you trust and ask which items are the most interesting and which should be omitted. Create a second draft based on this feedback. Determine the number of words to be added or deleted to arrive at a total of 300.