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.