First students took a look at the different templates to see which might work for the theme of their book. They then went to this site to print a planning template.
http://learninginhand.com/blog/2014/8/6/plan-a-better-imovie-trailer-with-these-pdfs
This is extremely important. There is no question the kids could have quickly whipped off a movie, but this forced the students to think of powerful images that represented the book.
Students then used the iPads to search images and used iMovie to create their trailers.