When I did mine, I installed the complete light assembly into the bracket first in order to ensure that the lens was perfectly centered in the tail panel, as well as deep enough to get a good seal on the gasket before I tacked it in place. Hope it helps.
EDIT: These are AMD pieces and required some trimming for a good fit in my situation.
Thanks for the pictures and info on triming them,thought I had hem right, the picture on auto metals website is upside down, and yes I will still have to trim them