Before using one of the PDFs on the Storyboarding page, it’s helpful to brainstorm what the trailer might include and then narrow down the content to essential and important details. Below is a link to a fillable PDF. This one is designed to help with brainstorming and/or researching possible content for the trailer.     After brainstorming and/or researching, the next step is to narrow down what content will actually be included in the trailer. The trailer must contain what’s essential. It should include as many of the important details as possible, and maybe just one of the interesting/funny/trivial information. Remember, trailers are very short, so you have to be concise.

Before you get to planning your next trailer, here are some tips:

1. Use as few words as possible. Trailers are fast-paced and text is never on the screen for very long.

2. Shoot or choose photos and videos in landscape (horizontally). Vertical videos and photos will have black bars on the sides of the screen.

3. Need to include more text in your trailer than what the template accommodates? Use Skitch, Phonto  or some other app to place text on top of an image. Splice for iOS is a great choice if you want to put words on top of a video (and you can of course do this in a regular iMovie project).

4. Looking for copyright friendly images? Use Haiku Deck. Insert the image you want into a Haiku Deck slide and take a screenshot. Don’t forget to give attribution for the photo.

5. Speaking of Haiku Deck, that’s another great choice for putting words on an image. Simply take a screenshot of a slide you’ve worked up in Haiku Deck and use that in your iMovie trailer project.

6. Movie trailers follow a very specific format. Each shot is on the screen for just a second or two, and never longer than 4 seconds. So if a movie trailer doesn’t fit your content, you should probably choose to make a regular movie project.

7. There are other apps that can scaffold your movie-making by providing a format to follow. These include Directr and Videolicious.

8. iMovie comes free with a Mac and you might be able to get iMovie for iPad for free. iPads purchased after September 1, 2013 and running iOS 7 or later are able to download iMovie at no cost. 

source: http://learninginhand.com/blog/2014/8/6/plan-a-better-imovie-trailer-with-these-pdfs