ok. here goes...
only the bottom half of the back comes off. And it is a pain in the arse.
Hold in both hands with the back facing you and the camera at the bottom. Put your thumbs at the bottom of the bit that comes off (by the dividing line) and your fingers on the front of the phone.
Push your thumbs in (there's a small clip in the middle that needs to be pushed inwards) and slide the back off the bottom of the phone.
It's difficult to start with, but once you've done it once, you'll wonder what all the fuss was about.