For install, ideally you'd want to use a template that would be supplied with a new spoiler - that's how OEM spoilers are installed on a Camry if you were buying it new and adding that option. Finding a template for a 2000 Camry is not gonna be easy.
The 4 bolts will hold the spoiler just fine - it's not like you're pushing hundreds of points of down force through there. Besides, the glue alone offers a LOT of support. I believe the square cutouts are there to aid with alignment more than provide support. If you really don't want to deal with it, just make sure you align all the bolts before tightening everything.
As for wiring, factory wiring runs the harness to the 3rd brake light inside the car and plugs into the existing harness replacing the 3rd brake bulb socket. An interior color flat cover then covers the hole in lieu of the 3rd brake light w/ housing you'd be removing. In your case, I'm assuming you're missing the said cover and will be keeping the 3rd light in addition to the spoiler light. You have two options in this case:
-Keep both lights. Looks kinda weird if you ask me, but it's a matter of personal taste
-Disable the stock light. You'll have to replace the brake light breakage sensor if you do this (blue box on D side in the trunk behind covering), or you'll have the idiot light on every time you hit the brakes. The sensor relies on detecting bulb resistance, and since the spoiler LED bulbs offer much less resistance, the sensor thinks your light is out. There was a different sensor for cars with spoilers that accounted for this.
Hope this helps.