First you will need to remove the horn assembly from the steering wheel. There are three, slotted screws that you undo from the back and then the assembly can be pulled from the wheel. There's also an electrical contact that needs to be unplugged.
Next, remove the horn contact plates from the horn cover. There are several screws that hold this in and, on mine, I also had to remove a little box that houses the cruise control switches. The contact plates can be separated by using pliers to pinch the plastic rivets and push them out. Once that's done, pry the two plates apart. There is a layer of old foam that separates them, and that is what needed replacing.
Then I used some fine, steel wool to buff it out and then wiped the residue off with a damp cloth and dried it.
Then I used 1/4" thick, 1/2" wide weatherstripping to add a new foam layer. It is important to make sure the foam does not cover any of the raised areas on the contact plate because these are what actually contact the copper plate to make the horn honk.
Once that was done, I reinserted the plastic rivets and- voila! The horn was repaired! And now I'd like to borrow a line from an automotive repair manual regarding rebuilding an engine "step #473: reverse procedures to reinstall". Isn't that helpful?
If you've made it this far in reading this post then you are either: a true friend, really bored or attempting to make the same repair. Good luck!