Make a note of all the footage or ideas you have for your movie. Having a physical log of your footage in front of you will give you a clearer idea of what you have to work with. It might be handy to make your notes on note cards.
Divide all of the cards into two piles: what's important to the main plot of your film, and what isn't. Leave all of the side plots or minor character moments for the audience to discover on their own. Your trailer is when you want to push the story of your movie. You need to show people your main draw.
Decide what the theme of your story is. This will not only help you pick footage to put in the trailer, but it will help you write voiceover copy as well.
Pick out what you want to be your opening and your ending. These are going to be the two most important images that viewers see. Your opening introduces people to your trailer and to your film in general. The closing shot is their last impression. The closing of the trailer can often be that final bit of information that helps a viewer decide whether or not they are going to see this movie.
Fill in the middle with exciting snippets of footage and dialogue that will not only build around your theme, but also keep a viewer's interest. Try not to give away too much of your story; you want to get people excited and give them something to look forward to.
Write voiceover copy that doesn't give away your story, but clearly expresses your themes. Allow the voiceover to guide your audience and make clear to them any story elements that you wish to convey.
Use snippets of character dialogue to illustrate and set up your movie's world. This will also give your potential audience the opportunity to actually hear your character's voices. Two characters talking is an efficient way to set up your movie's important relationships.
Choose music to set the pace of the trailer. When you're writing your trailer, consider music that you feel really establishes the mood of your eventual movie. Allow your imagination to wander and find the song or songs that you feel best highlight your story.
Interweave the audio with your images in a way that keeps the content of your trailer moving at a quick pace.
Place your scene heading first. Scene headings are also referred to as slug lines and they're used to identify whether the scene is outside or inside, the precise location of the scene and the time of day the scene takes place in. Inside or outside is referred to as INT (Interior) or EXT (Exterior). Place a period after INT or EXT ("EXT."). Place the precise location after the period (EXT. FIELD). Follow the precise location with one space, a dash and then another space (EXT. FIELD - ). After that space place the time of day whether it is MORNING, DAY or NIGHT (EXT. FIELD - MORNING). Use all caps so that people can easily make out the Scene Heading. You'll need a new scene heading every time your trailer changes locations.
Leave one empty line between the scene heading and the action. The action describes what people are actually seeing on screen. It's used to set the scene for what is actually happening while your characters are talking. It should be written with basic grammar and punctuation rules. Explain everything that your characters are doing except for their dialogue. In a trailer, you'll write a lot more action that you will dialogue.
Leave an empty line and tab over five times before inserting your character's name. Write your character's name in all caps so that it can be seen clearly by readers. You'll need to do this anytime a character is going to speak a line of dialogue. If it's just an anonymous voiceover, identify the character as NARRATOR.
Write your dialogue on the line directly underneath the character's name, four tabs over. Again, you'll need to do this anytime a character speaks. Trailers need to hold your audience's attention so if you write a lot of dialogue, write short and snappy lines.