Install is easy. Getting the cluster out and apart is just some screws. The overlay is meant to slip over the top of the old one without taking the needles off.
You just put all the needles straight up, and pull the overlay down across them from the top. It's pretty flexible and the needles have some give to them too, so there's not much worry of breaking anything.
You will have to wire up power and ground. I'll see if I can dig up how I wired it. I wanted it to come on only at night like it's supposed to, but I couldn't figure out which power wire to splice into. I ended up just picking a random one and it stays on all the time (when the car is running). You can't really tell it's on during the day though.
EDIT: Here's the thread I made back when I was installing it.
http://www.grandprixforums.net/diagr...tor-68760.html