I used nothing fancier than an anvil pruning shear...
To form a scarf joint where the ends meet at the bottom of the lens.(not nearly as steep as in illustration)
By holding the shears at an angle when I made the cut.
Also did the same where the horizontal pieces meet the vertical sides.
And yes, the perimeter is all one continuous piece, as said, jointed at the bottom/middle of the lens.
If you get the same stuff I got, (I bought a whole roll @ NAPA) It'll almost turn the sharpest radius. A little bit of the sharpest corner is visible behind the molding, but......I just blacked that out with a sharpie marker.
I found it easiest to remove the tail light, clean it up real well with dish detergent & water, then clean it again with a little rubbing alcohol on a rag.
Then, I put it in my lap & started cutting & forming, peeling & sticking.
Go to an auto parts store & ask to see their Trim Guard catalog to find a narrow 1/2 round shape...OR, there's always ebay...
http://cgi.ebay.com/ebaymotors/1-2-X...item2ea3a3d693