Write an outline. A short script is typically between 15 and 30 pages long, timed at about a minute per page of film time. Know your story so you can make effective use of this short format.
Format a short script in the same way you format a full-length feature. This means you'll be using scene headings and character and dialogue indications throughout. Movie Magic Screenwriter and Final Draft are screenwriting programs that do the formatting for you so you can focus on writing. You can also use a free online service (see resources) without the need to download any additional software.
Write only what you will be able to see and hear on the screen. Keep the action scenes active and use as few adverbs as possible. Don't write camera angles and other directorial elements. This will waste valuable space in your short script.
Keep your story concise. Focus on one story line. Anything written outside one story line detracts from the limited space you have to write a beginning, middle and end in your short script. Follow these simple steps and you can write a short script that tells a single, effective story.