I got my MMD Matte Black Taillight Trim from AM. It was super simple. You're right, it comes with 3M tape on the back, around the outside frame. It also comes with small alcohol pads to clean the taillight area before you install it. It's very obvious how they fit; mine sit perfectly in the indentation around the taillights. I wanted to be sure the posts separating the lights and the corners would stick, so I cleaned them with alcohol and cut small strips of 3M tape (which you can get at any auto parts store, or even Walmart) to fit. Then I used alcohol to clean the car body anywhere tape would touch. Finally I peeled the plastic backing off the tape, gently placed the trim where it fit, and when I was happy with the location I pressed down hard. I ran my fingers over every piece of trim to make sure the tape was really stuck on every surface, especially the corners.
I hope that helps! Like I said, it was very simple.