Something that drove me crazy was the dust/residue/stains on the inside of the camera protective lens. After all, this is my main picture phone! Here is what I ended up doing in the end:

0. Wait until you are ready to sandwich the back glass to the frame

1. Drop some alcohol into the protective lens box

2. Wipe down the lens with a lens wipe, pushing it in with a spudger

3. Drop alcohol AGAIN

4. Blow it out with a blow dryer

5. Quickly sandwich the back on

Crystal clear, no dust, no residue, no water stains!