Installation is actually quite easy. Of course I do all sorts of mechanical stuff, so I guess it could be just me.
Check out these instructions:
DIY: Roadster Spoiler Install
Just remember to be patient and measure, check, measure, check, measure some more, check some more. Look at how it sits around the trunk tail light. Measure that a couple of times. Then drill.
I also added a bead of silicone across the front just in front of the foam. Nothing that can be seen once it's installed. The idea was to keep water from flowing under the spoiler. I left gaps at the rear so if it did get in it'd be able to get back out. Three years and no problems yet.