Well, you don't. You write a few sentences. They make paragraphs and pages. Pages make scenes. Scenes make narration point of view, chapters.
Chapters add up and make a book.
When I sit down to write, I start out with sentences, searching for ways to write out an idea. Sometimes a sentence may have a character or characters in it. I write almost every day, although on weekends, I don't write as much or as often. During the week, I work starting about nine or ten o'clock in the morning. If you're a student, it helps to write a half a page or a page a day.