In this DIY post I'll walk you through how to install a wing on your car. For this post I'll be installing a middle hatch wing on my 03 Impreza TS wagon. It's the same technique I would use on the trunk lid of a sedan. This install takes about an hour to complete. Please bear with me, I had to attach all the pics at the bottom as I've been having trouble with my photobucket pics showing up on here.
Step 1 - ensure you have all the parts and tools needed before starting. Mine came with 6 bolts (3 on each side) to fasten it to the hatch. As for tools and supplies, you will need a ratchet/sockets, drill, drill bits of various sizes, Dremel grinding stone, tape measure, marker, punch and hammer, hand lotion or toothpaste (I'll explain later)
Step 2 - Measure to find the middle of the wing. This will make it a lot easier to centre on the car. Mine measured 137cm so divide in half is 68.5cm. Once you measure the middle mark it. I used cm rather than inches since it's a lot easier for me to use. But either form of measurement will work.
Step 3 - Take the lotion or toothpaste and dab it on the bolt holes on the wing. Then use the middle marking and line it up with a reference point on the vehicle. I used the rear windshield wiper and lined it up with the marking to center it. Gently place the wing where desired on the car. When you place it down it will transfer the lotion/toothpaste onto the car showing you where to drill your holes. I learned this tip on Mighty Car Mods when they installed hood pins. Works well for anytime you need to mark where to drill and it's nice because it's not a permanent marking if you mess up.
Step 4 - Measure, measure, and measure again to ensure that your markings are centered. If not, repeat step 3 until you are satisfied. Once everything is centered, use a punch to permanently mark where to drill.
Step 5 - As bad as this sounds, it's time to drill some holes into the body of your car. Start with a small drill bit to use as a pilot hole. once drilled, move up to the size bit you need to fit your fasteners through. Then finish off with a Dremel grinding stone to remove the burrs and sharp edges.
Step 6 - Installation. First you'll want to find a way to seal up the holes so water doesn't get in. I'm using 6 rubber hose washers (rubber drain plug gaskets would work too) and gluing them to the mounting holes on the wing. Use a rubberized glue to ensure a proper seal. I've heard of people using silicone and RTV gasket maker, which probably works, but would be really messy. Once your hose washers or drain plug gaskets are glued and dried, line up your bolt holes with the holes you drilled and fasten the wing using bolts. If you have an unpainted wing like I do, I recommend doing all this before painting it as there is less risk of scratching it. Once the test fit is complete, remove it, paint it, and reinstall. If you're going to be driving the car or have the car outside while the wing is in paint, make sure to cover up the holes to keep water out. I used water proof painters tape on mine, so it'll keep the water out, and won't leave a sticky residue when I remove it.
There is probably other methods for this as well. This is the method I used and I'm very happy with the result. Hope this helps, feel free to post up some pics of your DIY wing installs!