Don’t overthink it. The biggest mistake people make when they embark on a short script is they are thinking in the three-act structure, which is the feature film format. Even if you aren’t familiar with the three-act structure, if you have watched enough movies, you are very likely thinking the same way.
Feature films are complex with plots and subplots. A short film is a bite size piece of a feature script. It is a scene or a few scenes that introduce a character, a problem and a solution. It doesn’t have to be clever or have a twist. All it needs are characters we feel for in a situation that gets our attention.
Don’t cram a feature into a short. For example, instead of telling the story of a princess in a galaxy far, far away who unwittingly recruits a motley crew that rescue her from an evil emperor in a “piece of junk,” focus on a part of the story. A princess, pursued by bad guys hides important documents in a droid who escapes in a pod to keep them safe.
The best way to erase a three-act structure habit is to watch short films. Find out who won Sundance, Toronto, Tribeca or other big festivals to see what makes a good short film work. A lot of them are available online and if you have Amazon Prime, they stream a lot of award-winning short films. Once you have watched good examples of short films it will be easier to brainstorm ideas.