Remove the five 10mm fender bolts. Be careful w/ the ones near the door- don't ding your door!
With bolts removed carefully lift up the top of fender and pull it over the "hump"- while doing this pay close attention to the top fender portion near the door- the door ahs to be open a certian amount- again don't ding your door!
Once it's pulled over it will be very loose and you'll have plenty of room to work.
There are four 14mm bolts holding the oe piece in- remove these. Put the STi one in (match it to your oe one as they are right/left specific).
Torque to 51 ft lbs.
Couple of comparo pics- the oe stamped one is rather flimsy- you could bend it w/ your hands- the aluminum one is VERY beefy and won't bend.
Everything goes back together the way it came off, pull the fender up and over the hump- thread in the 10mm bolts in finger tight. Carefully tighten- do not over tighten these small bolts- the factory calls for 5.5 ft lbs- just snug them up nicely- again pay close attention to the fender in relationship w/ your door. Replace the fender well (I pull the two piece fastenrs apart- stick the one end in first, the "plug" portion- seem to go in easier that way.