How to begin a book?

I'm often asked "How do I begin a book?" or "When I start a new story, I feel really inspired, but somewhere along the way I lose inspiration or the story fizzles--what do I do?"
Getting past chapter three is a challenge for many people, including me. To keep going without getting permanently derailed, I always remind myself that IT'S OKAY TO WRITE BADLY. During a first draft, we're not trying for deathless prose; we're just trying to stay alive. When it comes to the first draft, perfectionism is death--save it for the final draft.
Just get in there, get going, and keep going! You can't write a book without writing it, and it will only happen word by word. Often, in a first draft, the words are nothing special. That's fine. Don't worry about it. Set aside obsession and attention to detail. Set aside criticism. Just write. There will be plenty of time later to go back and polish up what you've written. By then, you'll be celebrating the fact that you have something to polish!