I haven't written a full-length book biography, but I have included biographies in many of my books and articles.
Start by writing your own autobiography, just to get clear about the life you know best, and to practice.
Also, read excellent biographies, and see how it is done. There are many methods. Do you go with facts, or personal accounts? How do you deal with controversies? Find one to three biographers and use their method of thinking and presentation (not their style).
Then decide who you are most passionate. And decide why *you* want to tell their story: What is your perspective and message (which is not the perspective and message of the person you are writing about.)
Then write one article summarizing key moments in the person's life. Then write a few more about specific events, words, or actions of that person.
Then put it all together.
Go for it!