The 05+ STi's come w/ some nice under aero cladding- this helps reduce lift at speed (and evidently may even help gas mileage a smidge)
Here the pieces I'm talking about
Here the parts you need
The aero pieces are
56411FE000 and 56411FE010, you'll also need 18 plastic clips (order 20 so you have a couple spares) 909130059 and two 6mm bolts 5770KC000 (I'd order four to have a coiple spares).
In addition you'll need a drill, good asst of bits including 13/64ths and 3/8ths, center punch, sharpie (red worked good), a tap for a 6mmx100, duct tape
Pics of the aero piece, plastic clips and bolts