Read the introductions of several of your favorite biographies, making notes about what subjects the author covers. What style of introduction do you relate to? What information do you feel is important to address? Every author will answer these questions differently.
Brainstorm all the topics that you want to include when you write the introduction. Think about why you are writing this particular biography and what impact the subject has had on your life or the lives of others.
Write a first draft of your introduction, avoiding editing. Just let the words flow. By censoring and second-guessing yourself at this stage, you may be causing a problem that many people refer to as writer's block.
Read over your introduction, noting areas that you want to revise. Correct spelling and grammatical mistakes.
Ask a trusted friend or fellow writer to serve as a helpful critic, or beta reader, on one of your early drafts. The job of a beta reader is to read a draft and offer constructive criticism. A fresh pair of eyes can often catch mistakes that you may have missed or make helpful recommendations.
Continue the editing and revision process until you feel confident with your end product.