First, make sure you intend to install a wing, not a spoiler. Spoilers are another option for your vehicle, but this page doesn't inform people on how to install them. An example of a wing is pictured at the left.
After you have picked out a wing you like, checked it for factory errors (like twisting, length and centering errors, etc.), and concluded that it provides an effect level you are comfortable with (some wings generate a lot of downforce and could drastically change how your car handles at higher speeds), you can begin the installation process. (Note: If your wing comes with installation instructions, follow those instead of this article.)
Trunk-Mounted Wing
The most common mounting point for a wing is the car's trunk. First, measure your trunk and find the center.
Bolt-on
Next, measure the distance between the mounting points on the wing. Most have two brackets with two mounting points on each, so measure front-to-front and back-to-back. Measure out from the center point on the trunk, half of the mounting point distance outward, and you'll be able to mark the points to drill mounting holes in your trunk.
Glue-on
Find the center of the wing, and line it up with the center of the trunk. Apply glue if necessary, and allow to dry (without driving) for the recommended time.
Performance Tips
The further back on the vehicle the wing is mounted, the more leverage the downforce has on the rear wheels. When you take advantage of this, you can achieve the same effect of a closer-mount wing, but with a wing angle that is much less steep, which reduces drag.
Hatchback-Mounted Wing
Most hatchback wings are custom-made for each model, due to the limited effective designs for this type of vehicle.