Create a story that you can tell with toys, action figures or other inanimate objects.
Take pictures, each representing a fraction of a second. For example, put a doll at one end of a table and take a picture, then move it an inch and take another picture. Move it another inch and take a third picture. Keep doing this until you've moved the doll across the table.
Import all of the pictures into iPhoto.
Launch iMovie and import the pictures.
Click Media > Photos > Show Photo Settings.
Reduce the photo time (the slider with the rabbit and turtle) as low as it will go and click "OK."
Drag the photos into the project.
Add dialogue, music and sound or video effects.
Save your iMovie.