Script -- I start by writing a preliminary script, usually based on the book's cover blurb and any synopsis the author provides. The customer may help to revise this script before editing starts.  How closely the script can be followed is determined by what images and music can be used. Basically, a script is a starting point -- like the outline or first draft of the book -- and the finished video may only vaguely resemble the starting script.

Image acquisition -- Much of the time spent  on video creation is for image searches and graphics processing. My first effort is to find free and/or public domain images. Failing that, I search microstock image sites.  The cost of any images purchased is added to the cost of the trailer and the customer is consulted before images are purchased..  If a customer provides their own images, there is, of course, no charge for their use.

Image processing --Virtually all images have to be processed in some manner, even if it is simply cropping to fit the video screen. Images also usually require adjusting contrast, saturation, etc.,  in a graphics editor.

Editing -- the cost covers manipulating the timing of fades, dissolves, and other transitions, and pans and zooms. Editing also involves putting titles/text on the screen to accompany the images and tell the story, or adding a voiceover.. 

Videos can take two weeks to a month to produce. I consult with the customer throughout the editing process.