Carry out your research. This will vary depending on whether your subject is still alive. Talk to the subject if possible, and also to his friends, relatives and colleagues. Research the life of a subject who has no remaining relatives or colleagues, by using secondary sources such as papers, articles and other biographies.
Note any particular developments that were occurring in the period of time relevant to your subject. Ask key questions, such as whether your subject was particularly influential at this time, or whether others influenced him. Research some of these influential people, without becoming too waylaid from your story.
Structure the biography appropriately. Give a chronological account, emphasizing the factors in the subject's childhood that may have contributed to her interest in her future career. Alternatively, start with recent aspects about the subject and then return to her earlier years. Highlight key achievements or milestones in her life and career. Immerse yourself in the subject's world. Visit places that were important to her, and attempt to see the world through her eyes.
Obtain necessary permission or aim to resolve copyright issues before you progress further with the book. Research the prospective market for your book. You might have a ready-made market, particularly if your subject is, or was, popular with a group of people