Tap the camera upload icon in the bottom left-hand corner of the screen and choose your favorite Live photo.
The date the photo or video was taken will automatically come up when you upload from your camera roll — you can delete it by holding it down and dragging it to the trashcan.
Hold down the image for a moment. A circle will appear briefly before it says BOOMERANG across the screen, and your picture comes to life.
Hit the "+ Your Story" icon, like you would for any other picture or video, and add that new Boomerang you didn't even mean to take.