How to write a novel

How do writers come up with story ideas and turn them into novels?

“If one idea in particular seems attractive, and you feel you could do something with it, then you toss it around, play tricks with it, work it up, tone it down, and gradually get it into shape. Then, of course, you have to start writing it. That’s not nearly such fun – it becomes hard work. Alternatively, you can tuck it carefully away, in storage, for perhaps using in a year or two years’ time.”  Agatha Christie