The writer must be in it; he can’t be to one side of it, ever. He has to be endangered by it. His own attitudes have to be tested in it. The best work that anybody ever writes is the work that is on the verge of embarrassing him, always.
Start early and work hard. A writer’s apprenticeship usually involves writing a million words (which are then discarded) before he’s almost ready to begin. That takes a while.