The instructions were clear and I managed to change the battery. Removing the old battery was the biggest hurdle. The whole operation took me maybe 45 minutes as I tried to be careful and also tried to clean the phone as I worked on it.
However, the replacement didn't fix my phone. The old battery pack of my 3GS was spent and the phone only worked when plugged in until it finally completely died off and would not hold charge or turn back on, just showing the empty battery/charging icon and occasionally the Apple logo before switching off again. With the new pack, this problem persists. The battery appears empty, phone will not turn on. I suspect the replacement battery may be a lemon. The battery heats up while charging, but so did the old one.
The problem is not in the charger/cables as I have tried many. The phone also has had problems with the mute switch and power button already before the battery started dying. I cleaned them the best I could but the power button still won't work except occasionally.