On step 3 when replacing the back panel with a new one, they aren't kidding about removing ALL the protective slips. There's THREE of them.
1), the obvious back slip that protects the new back.
2). the one inside the back protecting the side with the camera lens
3). the little square one OVER the lens itself.
First time I tried to reinstall my back, that tiny little square over the lens prohibited my ability to actually get the back on completely. My husband jammed it for me, but the screws still wouldn't take. It's amazing that these things are so precise that the tiny little film on the lens would make it that much tighter.
Anyways, hope that helps somebody. :)