Open iMovie
Go to File/Import/Movies
Choose Optimize Video: Large (NOT FULL)
Browse And select the movie or movies you want to import.
After the movies are imported go to File/New Project
Choose Trailers and preview a few until you find one you like.
Choose your trailer and click Create.
Go to the Outline and replace the original text with your own.
Go to Storyboard and select the sections of the imported video that you want to use for each clip.
Replace the original text with your own.
When you are finished click Watch Trailer or Done.
Go to Share/Export Using QuickTime.