When you write a script for a short film do you also write a script for the trailer?
Usually a theatrical trailer is prepared after the fact from the completed film. However, I can imagine if you're trying to get crowdfunded to produce your short, your question might be "How do I take my script to make a teaser to interest potential investors?". And I believe some online distributors like to have a trailer even if it's just a short.
If you look on IndieGogo or Kickstarter, you'll find a variety of techniques from simple storyboards up to animations and clips. It really depends on where you're starting from financially. Having a teaser clip is helpful. Often you need to spend money to make money.
As a general guide, you want something that introduces the situation and the predicament. This often means a montage from your first act and an action sequence from the second act. Some trailers will also toss in a few un-narrated clips of the final struggle from act three. As they also say, you don't want to give away the money shot, so be selective of what you put in. What you want--whether for theater or fundraising--is for the audience to be wanting to know more (Who are these people? How did they get into this situation? OMG, how are they going to get out of it? ... and if you had great special FX, "Whoa, that looks so cool!").
Often trailers are about visual action and less about dialogue. A picture conveys far more, so use the montage to your benefit. Pick four scenes which introduce the protagonist, the antagonist, the situation, and a crisis as the basic set. Sometimes you can use actor voiceovers/dialogue from one scene overlaid on alternate images. Using narration sometimes gives a retro feel unless you're doing so in your own recorded sales pitch but it can work.
Having a good editor is a bonus! Putting together a good trailer is more of an art. While it may be scripted, often it's culled together by an editor in post.
