I just replaced the lcd screen and the lens on my 8100. the face of the phone, when separated from the rest of the parts has the lens on it. It sits on the outside face of that part, and is stuck there with adhesive.
The replacement lens I received had some type of sticky thin foam on the back, I took that off and carefully stuck the new lens right where the old one was. When I took the old lens off, the body of the phone still had plenty of stickiness left to it. Replacing the lcd screen was easier in my opinion than replacing the lens. The screen has a definite "click" when properly installed. The lens is a bit trickier. Also be careful to not get any marks on the inside of the lens. I had to take mine apart twice because there was a speck of dust on the inside after first putting it together. I didn't wear gloves, but just be careful. The face of the phone looks brand new now.
By the way, when I ordered the replacement LCD screen, I also ordered the tiny flathead screwdriver (along with the tiny torx driver and case opening tool from pdaparts website). That tiny flat screwdriver helps alot when disassembling the phone.