Gather your information via interview, research or from your own memory.
Choose a tone. Is your subject a serious person, a comedian or a criminal? In such a short body of work, your tone needs to be obvious, as does the subject's personality.
List the three most noteworthy or personally reflective events, facts or trivial anecdotes regarding your subject.
Write those three events, facts or anecdotes into a three part thesis statement. For example, "John Smith is a new father, home brewer and tamer of rabid chihuahuas." Keep in mind the tone you want for this mini biography.
Write the rest of your mini biography as if you were writing the introductory paragraph for an essay about the person's life. Give slight elaboration on your three part thesis.