Research the subject of the biography thoroughly, using both primary and secondary sources. While stories from friends, family and colleagues can offer a glimpse into the person you will be writing about, don't assume they are also completely accurate. Whenever possible, back up stories with facts, and always note when something is merely opinion, rather than a fact. If you're writing about a particular U.S. senator, for example, review official papers and documents that he wrote or worked on. Often, during this process you will have to be half writer, half detective.
Convert your notes into a series of topic-based index cards. For example, the index cards for a biography of Abraham Lincoln might have a section devoted to his early law career in Illinois, while another section might cover his family life. On one side of the card, write down the topic of the card and the source for the information, while on the other side write down the information itself.
Write a chapter-by-chapter outline, and within each chapter list topics you wish to cover. Here is where the topics side of your index card come into play. Don't worry if you start to feel that you have too much extraneous material; you will make changes with each subsequent draft. Early on you just want to concentrate on getting your ideas and facts down.
Start writing the biography by filling in the pages with the information on the other side of the index cards, adding your own evaluation of the the topics as you write. Try to weave your notes together into a coherent narrative.
Use quotes cautiously. When you quote, it's important to use quotes marks, but give proper citation for them. For example, you might say that a specific speech or literary work played a key factor in the person's career choice, and decide to include a quote. Always double check to make sure you got the quote right.
Whether you decide to use footnotes or endnotes for your biography, be extremely careful to to give a full and clear citation for every quote, idea, or reference you take from any other source. Several biographers have gotten themselves in trouble by failing to do this. Also, use the guidelines from the Chicago Manual of Style when citing. And there you have it. When finished, you will have a rough draft of a biography that is the first of many edits.