Writing the biography of a living person is similar to writing that of a person who has died. Many of the considerations are the same, such as major life events, influences and relationships. The major difference is that when writing about a living person, you must consider what he will think about your work, and you also have access to his personal voice and sway.
Contact the subject of your biography via email, agent or telephone. Schedule an interview. If you cannot schedule an interview, make a plan for extra research.
Research your subject on the Internet and in books (if possible). Interview her relatives, friends and anyone who knows her. Formulate questions based on events in her life, her interests and her relationships. Ask about the subject's feelings, reactions and how events in her life progressed. Interview people who don't know the subject, but know of her, and get their take on the biographical details.
Create a timeline with the events and transitions of the person's life so you can visualize the progression. Add to the timeline the details you obtain from the interview and your research. Decide how you'd like to organize the biography--chronologically, by events, or some other way you decide.
Write the biography. Ask for input from friends and advisers to help logically organize your work.
Edit what you have written. Give it to other people to read to help point out mistakes and smooth the writing.
Interview as many people as you can, and consider different perspectives of evaluating and describing the person's life. Keep your notes organized so they don't become overwhelming. Keep in mind your structure throughout your work so that you don't get confused and inundated.
Remember to consider the feelings of your subject as you write; he still alive so he will have access to your work and will likely have a reaction. If you know that what you are writing will not cause a positive reaction, be ready for the repercussions of your opinions and representation of this person.