Sentences and stories have a texture and for me the process of revision is essentially the process of smoothing away all the jags in the texture of the prose. Do this over and over and the prose starts to feel water-worn and smooth, and in the limit I have smoothed away my own imprimatur—what is left feels like no more than the most natural way of ordering words. The goal is for the prose to be so light that it is almost unnoticeable, that, in the ways the connotations and denotations merge and interact, it has a sense of opening a window directly onto meaning. Interestingly, to do this well, I think, one must consider the rhythm or music of the words, which has only a peripheral relationship to meaning.
So much for revision. Where do first drafts come from? I suppose there is generally a seed—a sentence or a sentiment or an idea of plot or perhaps just a constellation of elements—and other ideas accrete around the seed, and then around the secondary accretion, and so the piece burgeons.