Ted Geisel (a.k.a. Dr. Seuss) had this to say about the writing process when he discussed writing the very first Beginning Reader -- The Cat in the Hat.
I spent half a year working [on this limited word story].
When I came up, I solved my problem by writing "The Cat in the Hat." How I did this is no trade secret. The method I used is the same method you use when you sit down to make apple stroodle without stroodles.
You forget all about time. You go to work with what you have! You take your limited, uninteresting ingredients (in my case 223 words) and day and night, month after month, you mix them up into thousands of different combinations. You bake a batch. You taste it. Then you hurl it out of the window. Until finally one night, when it is darkest just before down, a plausible stroodle-less stroodle begins to take shape before your eyes!
Since "The Cat" I've been trying to invent some easier method. But I am afraid the above procedure will always be par for the course. At least it will be just as long as the course is laid out on a word list. (New York Times Book Review, November 17, 1957, Quoted in the book by Caroline M. Smith, Dr. Seuss The Cat Behind the Hat, revised edition. Chicago: Chaseart Companies, 2012.