You just keep writing.
Just grind it out. The next day re-read what you wrote. It may sound better than you thought it would. If not, you'll probably have some idea on how to make it better. But don't worry about it being perfect. For one thing, you can rewrite and polish later. For another, nothing is perfect, so don't expect to write the perfect book.
What you can do is write the best book that you can write. It will be your idea. Your voice. Your story.
That wonderful book that you had in your mind before you ever sat down and actually started writing is never going to be perfect.
But don't give up. Just keep writing.