1) Brainstorm Individual Ideas For Potential Movies. This can be completely original, or based on a book, movie or film you have already seen. The choice is up to you. 

2) Pre-view the iMovie Trailer Genres and decide which one fits your final idea the best. Pay special attention to the music and text style. Ignore the generic apple "stock vacation /event family footage" as you will be replacing it all with your own awesome clips.  
Tip - check out this trailer addict site to preview professional trailers. Take note of how many cuts/shots there are as well as the pacing, music etc. 

3) When you have decided on the genre/trailer style that you want to use, go to This Site. 
Find the trailer style you want and print a copy. 

4) Fill out in detail the iMovie Plan Form using it as your storyboard and shot list. 
Fill in a visual description, and indicate the action taking place, the specific location it is taking place, and what props/costumes needed.  Change the text etc. This is your blueprint and needed to film.