When Crashing was named a finalist in The Lascaux Prize for Flash Fiction, I was asked the above. So:
As I work, a section will leap out as a possibility because it works regardless if the reader knows what came before the section or what comes after. The section can offer a beginning-middle-end, or an image that drives several pages or graphs, as is the case with Crashing.
In adapting, don’t be shy about pulling from other parts of the narrative to make the short work. The pitfall to avoid is trying to tell the entire novel in the flash or short story. Let the little thing fly, even if ultimately, you allow dissonance between the story in the novel and the story that emerges from the excerpt.
I think about the opening graph of Hemingway’s The Sun Also Rises. A complete story, a man’s whole life in one graph, yet the novel proceeds for chapters and chapters:
Robert Cohn was once middleweight boxing champion of Princeton. Do not think that I am very much impressed by that as a boxing title, but it meant a lot to Cohn. He cared nothing for boxing, in fact he disliked it, but he learned it painfully and thoroughly to counteract the feeling of inferiority and shyness he had felt on being treated as a Jew at Princeton. There was a certain inner comfort in knowing he could knock down anybody who was snooty to him, although, being very shy and a thoroughly nice boy, he never fought except in the gym. He was Spider Kelly’s star pupil. Spider Kelly taught all his young gentlemen to box like featherweights, no matter whether they weighed one hundred and five or two hundred and five pounds. But it seemed to fit Cohn. He was really very fast. He was so good that Spider promptly overmatched him and got his nose permanently flattened. This increased Cohn’s distaste for boxing, but it gave him a certain satisfaction of some strange sort, and it certainly improved his nose. In his last year at Princeton he read too much and took to wearing spectacles. I never met any one of his class who remembered him. They did not even remember that he was middleweight boxing champion.The Sun Also Rises by Ernest Hemingway