How to replace a taillight lens?

[Dave 82 Heritage]

I need to know how to get the existing, broken taillight lens out of my 82 Bird. any help would be greatly appreciated! I have a replacement lens but can't figure out how to get to the old one. I know once I get to it I have to carefully break the silicone seal and pop the corners loose. Just don't know how to expose the one in the car. Thanks, Dave.