I can't add much and I am by no means an expert but I do know the Unity spot lights come with a kit for drilling the hole.
http://www.unityusa.com/Spotlights_c_98.html It also comes with a bushing that once you mount the exterior bracket, put the bushing in the bracket and drill the hole. I wouldn't attempt to do this free hand for the obvious reasons.
The spot light you purchased should have come with a specific kit and instructions for your vehicle.
