Press and hold a finger to your screen. Don't tap! That will bring up the text tool. After pressing for a couple seconds your screen will go dim and your story will go into Boomerang mode (signaled by "BOOMERANG" popping up on screen). Once that fades away, your live photo will be a moving Boomerang! Now you can edit your story to your liking. GIFs, music, location tags, whatever strikes your fancy.
Image Source: POPSUGAR Photography / McCall Minnor