Here's a pretty in-depth write-up (with pics) about changing your coolant:
How To: DIY Engine Coolant Change - IWSTI.com: Subaru WRX STI Forums
Your coolant system should hold about 2 gallons of fluid. If you just want to top it off, your safest bet would probably be to go to your local dealership and purchase 2 gallons of pre-diluted Subaru OEM Coolant. If they only sell the undiluted stuff, you'll only need 1 gallon of that - but be sure to buy 1 gallon of distilled water and mix them both together. Now that you have 2 gallons of diluted fluid, wait until your car is cool and take off the metal fill reservoir cap (the one next to your tmic) and add the fluid there (because it's the highest point in the system) until it almost reaches the top of that reservoir. Leave the cap off, turn your car on, let it idle, and watch for bubbles to disappear in the reservoir. The car should idle until it gets up to your normal running temperature (probably about half way). It's a very good idea to have a second person keep an eye on the temp gauge to make sure the car doesn't get too hot while you keep an eye on the disappearing bubbles in the reservoir. You might have to slightly rev the engine (to ~1,500 rpm) for about a minute or two to get the car up to temp. Once the car gets up to temp, the fluid should start to cycle through the system and the level in the reservoir should slightly drop - add fluid as necessary. Once the bubbles are gone, put the cap back on and go for a short ride. Again, keep an eye on the temp gauge and stop to let everything cool if it gets too hot (if this happens, you probably still have some air trapped in the system). If it heats up, pull over, let it cool, remove the metal reservoir fill cap, turn the car back on, watch for (especially the big ones) the bubbles disappear, replace the cap, and go home. If everything is ok, go back home, let it cool, and top off the fluid in your fill reservoir as well as your plastic tank by your radiator - it is normal for both levels to slightly drop after the test drive. That should do it. Depending how empty it was, it might not be a bad idea to add a bottle of Redline Water Wetter too. I'm a fan of the stuff. You can just put that in any time. Good luck!
Edit: I wouldn't add a bottle of Subaru Cooling System Conditioner unless you're completely draining/flushing the system. The above process should keep your car in good shape until your next scheduled coolant service.