Make sure to have something to pound the new seal end evenly. The seal has a nice little coating that squeezes in pretty tight and comes off easy. A nice size socket, or borrow one of those seal install tools from autozone...
oh and make sure to mark your driveshaft so it goes back on balanced right.
.....and most imprortantly, make sure the rear of your car is elevated higher than the front. Otherwise, you're going to end up with a good portion of your transmission fluid on the ground and all over yourself while you're trying to put the new seal in.
Thankfully, the fluid doesn't come out THAT fast, but it does drip...
Since the seal is larger than most people have sockets for, you can also use a small piece of wood, or hard plastic that covers the entire seal.. Just beat on in med. hits, evenly around...
I had thought of this though when I was reading this, your car seems fairly new (if I read 2003 correctly), I wonder why it's leaking so soon...
My last tranny hadn't blow a seal period for nearly 150,000 miles of continuous beating.. Maybe it just went bad?? Do you have any driveline vibrations when you're driving??