First off here is a picture of what happened to my stock pulley.  As you can see the pulley actually separated.  At first it didn't appear to make any damage (boy was I wrong).  :(

Getting Ready

The first step is the obvious one.  Ensure you have the proper tools to complete the job.  These are pretty standard, with only one exception.  The required tools are:

1. Basic socket set, wrenches and extensions (mostly just a 12mm, and a 22mm)
2. Large flat blade screwdriver
3. Torque wrench with a minimum strength of 115 ft/lbs!
4. New Belts.  The belts they recommend DO NOT FIT!!!!  They are much too big.  It took me some trial and error, however these one's do fit.  Gart #040320 (A/C) and a Dayco Dynaflex 335K5 (Alt / PS).  I was unable to get a Gart belt the right size so I was forced to mix and match.
5. Pulley (obviously)  :)

Removal of the current belts

Start with the left side (outer belt pulleys).  This belt controls your alternator and power steering.

1. Loosen the lower nut (right on the pulley).  This bolt takes a 12mm socket.  (left picture)
2. Loosen the screw looking bolt.  This will loosen the belt so that you can remove it.  (right picture)

The right side is equally as easy.  This belt runs the A/C compressor.

1. Loosen the top nut.  (12mm)
2. Loosen the lower nut.  (12mm)  The belt should now come free.

Crank Pulley Removal

Now for the hardest part of the of the installation.  Not exactly like its difficult...just its one stubborn bolt.  To remove the pulley do these steps:

1. First off, crawl under your car and locate your flywheel.  (This is where its much easier to have two people to do this.  As you will need to be under the car and the other person will be loosening the main bolt.)  Once you found the flywheel, you need to insert a large flat screwdriver into the splines.  Hold this here while your friend takes off the pulley bolt.  HOLD TIGHT!  This will keep the engine from turning.
2. The other person (unless you figured out a way of keeping the flywheel from turning), take off the main bolt with a 22mm socket.

Bad Bad Pulley!!!

Here is the awful result of my crappy pulley coming apart!  Please note...yours should not look like this!  :)  As you can tell, my pulley ate completely through my timing belt cover.  You can also see that the pulley came completely apart in my hands as soon as I removed it.  Oh well, this give me the opportunity to explain a timing belt change with some pictures.
 

Installation

Simple just do the reverse!  :)  Make sure you torque the pulley bolt to 115 ft/lbs!  If you don't tighten it to this spec the pulley may come off!  NOT GOOD!

The Finished Product

First off this is the damage from the old pulley.  I was quite surprised when I looked it compared to the new one that cost me $90.  :(  One thing that has been quite useful, is the fact my Subaru dealer has ALWAYS had my parts in stock.  If you need a good dealer in the Colorado area, go to Burt Subaru on Broadway (they have also done some really good work on my car too).

Here it is.  Fully installed and ready to go.  Looks pretty good I think.  I can't really see a huge power improvement, however there is a noticeable difference in how fast it revs up.  You really notice it up in the high band, revs much quicker.  Since I had to change mine anyways, this was well worth the money and trouble.