Once you have the back plate in the proper orientation it needs to be pressed into place. I found it impossible to perform this by hand. Eventually I found a short length of 3/4" PVC pipe.

Align the pipe with the back plate.

Place the watch and pipe in a vise. (Soft cloth used to protect the watch face). Slowly tighten the vise and you'll hear a pop when the back plate snaps into place.