First, throw out the idea you need to tell a whole story in a paragraph. You're not. You're just sharing a complete moment, or the clearest fragment of one. My favorite ones feel like a single, sharp breath held—everything is already in motion when you start, and the ending isn't really an ending, just the natural point where you exhale. I always look for the one concrete, sensory detail that carries the weight: the sound of a key in a lock that shouldn't be turning, the particular chill of a hospital waiting room chair. Everything else exists to orbit that detail.
Plot gets condensed into implication. Instead of 'he was betrayed,' you write 'the note in his own handwriting confirmed it.' The character's history is the slight tremor in their hands, not a biography. The conflict is the space between what they say and what they don't do. It's less about building a world and more about choosing the single crack in it that shows everything crumbling underneath. I find the most effective paragraphs leave me with a question I can't quite formulate, not a neatly tied-up answer.
What usually ruins these for me is trying to cram in a twist. A sudden 'and then she was a ghost!' feels cheap at this scale. The revelation should be quieter, almost missed—the realization that the character has been talking to an empty chair the whole time, or that the winter light hasn't moved in the description. The ending line should feel inevitable, not surprising, like a door clicking shut you heard coming from the first sentence.