It was all done by my loacal Alfa dealer, however, I did see their process. They took the original bumper off, sandblasted it and rubbed it down, then they took the bottom secion off... very carefully! Once this was ready they took the new bottom section and tried to mate it to the rear bumper, but it didn't match up perfectly so they had to rub it down on the join and get a nice flat mating surface. Then they bonded the two peices together, using G-Clamps all over the kit to hold it in place so it would cure.
Once this was all done it was a matter of preping the whole unit, spraying and finishing. Tedious job, but, as you said, the finish is great.
Where has he been?