Hi Jim,
just struggled with this myself a few seconds ago and went onto this site to find an answer. someone who asked a similar question bought a tool to snap it back together without hurting the front face. i finally got the back of my watch on by covering it with a thin piece of felt and using a pair of flat pliers to just squeeze the back on until it snapped into place. thankfully the front did not break or scratch b/c it's not my watch :)
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix wrist watches.