How do you plot a short story?

Set stakes according to the wordcount

Every story has something at stake for the character. In a large, multi-book series it may be an entire kingdom at risk. When it comes to a small story, with a small wordcount, look for small stakes. Perhaps a free coffee is at risk of being given to someone else, or an embarrassing secret might get out. The smaller the stakes, the easier it is to flesh out a short story.

Match the character to the stakes

With a prompt, you are given a good direction on what is at stake in the story. Think about what kind of character would create the best tension in the situation the prompt gives. For example: if the story is about a cooking competition, what character would create the best tension? For drama, it would be someone viewed as an outcast to the cooking scene. For comedy, maybe it's not even a cook at all, maybe a car mechanic got lost and ended up on stage. Either direction will create tension between the characters and naturally lead to the simple Intro-Conflict-Resolution arc.

Get to the moment of change.

A story, in its most basic form, is just a character in a moment of change. Once you nail down who is going to change and how they are going to change... get as close to that moment as possible while still giving the audience all they need to know.