These are the steps for replacing the battery on a 2004'ish Lexus GS300. The only tools you need is a small Phillips screw driver and a new battery, CR1616. [flickr]4413246177[/flickr]
This is the only tool you need.
Flip the key over and undo this screw.
This is the only hard part -- use the tip of your screw driver to pry, twist a little and at the same time push the top of the case back away from the key. The top part should pop off with little force.
The reason you need to push back a little is because of this "lip" at the back of the key that is holding the top down.
Pop the actual electronics module out. You can see why Lexus build great cars. The attention to details os amazing. They put the battery type on the case itself in case you lost the battery. The battery type is CR1616.
Undo the two screws on the electronics module, and flip open the cover. You can now replace the battery. Note that these two screws are of different length from the first one.
And you are Done !
I ordered my battery from Amazon. It is only a few dollars and gets shipped right to my door.