Do Your Research
The first thing you want to do when writing a short film, is to get together a list of at least ten shorts that are similar to yours. Find films that are in the same genre and share the same sensibilities. Then watch them, study them, and break them down in order to see how they really work.
A good way of doing this is by writing an outline of the film as you watch it. This involves simply typing out what happens in each scene as it occurs on screen. Write a couple of sentences describing only the most important beats, and by the end you will have an outline of the entire movie that you can use to study and break down.