After practicing a bit with the welder, I decided to put the torch on the car this weekend. I picked about as inconspicuous a place as I could find: the openings around the taillights. The part I decided to work on first was the little square there on the bottom. I started to work on it with a brass brush to find good metal, and it turned into a bigger spot than I had thought, so I decided to cut the rust out and make a patch. While i was in there, I cut out the rusty metal in the trunk, too, so I'll be patching that up pretty soon.
For the patch, I just used a piece of scrap that I had cut out of the trunk and bent it in my vise. It came out pretty good. The weld still needs to be ground down on the outside corner, because I can't quite get the angle grinder in close enough. I'll have to get a carbide burr for my die grinder. But really, when the car is put back together, this is all under seam sealer and behind the bumper, so it doesn't really matter.