Here is the thread with instructions and video guys. I must warn you to do this only if you don't mind the risk of hacking the tail lights, it takes a lot of prctice and you must have very steady hand and great focus to keep your cuts precise. The guy in the vid is Chris, he is a master at light modding, and has built some serious head lights and tail lights for various cars, even many GT-r owners here in socal go to him for light projects, you can check out his website for more vids and pics.
To be honest, the z tail lights are much more expensive and harder to source a used one for cheap if you mess up, IMO, Op you're better off selling the old ones to offset the cost of your new tails.
DIY - Cut open OEM tails for modding - Genesis Forum: GenCoupe Hyundai Genesis Forums