Ok, it took me a LONG time to figure this one out..

Slide on one side of the plastic tab strip inside the rubber bumper.
Attach the other rubber tab strip to the car only.
Now attach the plastic tab peice with the bumper attached to the car.
Line up the rubber bumper over the plastic tab peice already on the car
Hammer it on.

I know it sounds confusing, its the best I could do. Put one side of the bumper on, assembled and assemble the other half while its on the car.

John