This is the guide I used. It's for a GT-S but it was basically the same for my GT.
http://www.newcelica.org/other/insta...ncai/index.htm
One thing, when you cut the wheelwell plastic, be sure to cut it away about an inch or so. There have been instances of people not cutting it enough, and it touches the intake in places. After time, the vibration from the motor has rubbed right through the intake piping, so be aware of that. Also block off the holes in the wheel well. Here's some directions on that.
http://www.everythingcelica.com/ubbt...psed_2__1.html
I bought a really cheap ass black school binder. The REALLY cheap kind which is only plastic, not plastic covered cardboard. Then I filled the holes with that.
Good luck man. Post pics when you're done! :D