All of Bummer's tip above are great, especially about adding a bead of silicone along the upper edge of the spoiler. I did the same thing, that will keep water from draining down through it. There have been members that have gotten water to collect in their spoilers.
Here is another good DIY for the install.
DIY Roadster Spoiler Install with Pics
Here is what I did to adjust for level & centering.
Photo 1 I used a string & judged the level of the spoiler with the 3rd brake light.
Phot 2 I judged the centering by the distance on each side of the brake light. It looks a little off in the photo but it's just the angle of the photo I think.
Be careful if you decide to have it installed for you. There have been reports of members not having a good outcome. Not level & or centered. Choose a good shop. Some don't really care too much.