Outline the preview. Since it is not yet shot, you will create a mini-version of the screen story to attract interest from a producer, director or even a panel of judges at a screenplay competition. While the preview may only last 30 seconds to one minute, you should still think about a beginning, middle and end.
Select the best segments. In the short time you have, you want to write a preview that will show your main character in ordinary life, moving into a challenging situation and struggling to overcome the odds against him. Highlight specific scenes, depending upon the type of movie. A comedy preview should include some of the funniest moments. If you're writing a horror script, include one or two of the best scenes that will make the audience jump.
Write using scriptwriting software. That will do the formatting for you, leaving you free to concentrate upon writing. Since this is only a preview, it will contain little dialogue. Movie previews should highlight action. A good way to format a movie preview is with a series of shots or a montage. You can create any of these elements with any screenwriting software. A series of shots will look like this:
SERIES OF SHOTS
A) An airplane blazes as it careens toward earth
B) Survivors of the crash make their way through the jungle.
C) A lion the size of a car LEAPS from the jungle.
END SERIES OF SHOTS
The shot series is a great way to cover a lot of action in a little space. The difference between a shot series and a montage is that the shot series covers related shots, often in one scene, while a montage covers a wider time period.