If you go to
http://www.mopar.com/perf_pt.htm you'll find a link to the install instructions down the lower left side.
I installed S1 myself, but I have an '04 so it was just a handful of bolts holding the PCM on. For your '03 you'll need to install the injectors also, so it's a bit more involved. I'm inclined to believe that it's easier than the instructions lead on though, 'cause mine was almost stupid easy. If you are at all comfortable wrenching on your car you can probably do it.
Once it's in and you do the quick learn for the tranny (also outlined in the instructions) you'll be hard pressed to wipe the grin off your face![8)]