Here's what mine looks like.
Basically, each clip uses 2 screws:
Image 1 & 2:
The first screw holds the clip on your bumper (you can use a screwdriver to drive it in, but be careful of the placement), and passes the big hole through to the small hole (which is the side that will actually hold the clip to the bumper)
In the first 2 pics, you can see the back of the clip (which faces the rear of the car), where it is screwed into the soft black plastic of the rear bumper.
Image 3
The second screw is put in after you pull the diffuser edge over the long end of the clip (the diffuser tip has a little ridge that should slide right over the bracket/clip)
Image 4 & 5
These 2 pics give you and idea of the locations of the 2 screws
It should look something like this (pardon the crappy MS paint):
