you can use Fusor and some mesh like you would fiberglass too. you also have to trim a little of the metal bumper underneath the plastic and cut out some foam. getting it to look right with filler was the hard part.

 

garagebuilt,
I'll be working with an old friend, Billy Bondo, who's been working with bondo (literally)since it was invented. He will have no trouble at all getting the final finish on the bumper. We'll experiment extensively on the donor bumper before we start the actual project. We'll be sure to use the flex additive in the primer and final paint.
I found a video on how to do it, looks pretty straightforward...
Mo