Before doing anything, view book trailers online, especially those in your genre. You can do a Google or YouTube search for your favorite titles or authors. You'll find one thing in common, they're all pretty short. They range from one to three minutes on average. Watch what they use for imagery, colors, music background, narrative and pacing. Take notes on what grabs your attention.
Write a summary. Determine what the most poignant lines on the back cover of the book you want to publicize and what makes you excited about the book. You need to grab the viewer of your trailer emotionally. You need to wow them and you only have a minute or two to accomplish this. Use excerpts from any testimonials or endorsements you've received. Plot out what you want to see narrated or appear in title frames.
Script any narrative you want to include in your trailer and decide whether you want to include a live passage from the book, like a clip from a bookstore reading or whether you want to narrate a salient point or two that makes the book thrilling or different. Make clear your genre and include at least one compelling character. Take your concepts and sketch them out in boxes or frames on paper. This is a process used for everything from TV commercials to full-length movies and animations. Begin timing each segment and get an estimate of your length before starting production. Remember your call-to-action at the end -- direct the viewer to your website, blog or online bookstore(s).
There are a host of programs you can use to create your book trailer. Windows Movie Maker is a good place to start. There are also free and reasonably priced programs especially designed for the purpose. You may also want to incorporate background music and royalty-free images to illustrate your narrative. Both of these can also be had for free or at a modest cost online. Pans and wipes add motion and help keep interest when you are using still images or titles slides. If this process sounds somewhat daunting, consult friends who might have video editing expertise or hire the services of a professional.
Review your finished trailer. Make sure the narration is loud enough and that the background music isn't too loud. Look at the pacing and flow of the trailer. You don't want any dead air. Allow plenty of time for viewers to read title slides. Try not to exceed a half dozen words on any given slide. Make sure your transitions are complete with no overlap onto the subsequent slide. When you're satisfied with the final version, save it in a 640 X 480 format as a .wmv file. Now upload it to your YouTube account (they're easy to create if you don't have one) and copy the link to your website, blog, Facebook, Twitter or anyplace else you can think of to publicize the book.