Think about the basic structure of short fiction: opening, background, rising action with conflict, climax, and resolution. You will either clearly mention or imply these elements in your one-sentence narrative.
Play around, writing sentences that either imply the presence of a central character, as in Hemingway's story, or that clearly mention a character. Rather than think through an entire plot, go straight to the result of the climax: "For sale: baby shoes." Add the clincher that implies that a drama has ensued and ended: "Never worn."
Punctuate to craft a single sentence. To present your narrative as a single sentence, don't use periods between independent clauses. Use colons or semicolons, or add a coordinating conjunction (and, or, for, nor, so, but, yet) and a comma between them.