First off I can say that I have done something like this, but with the ignition and not the horn - my friends ignition switch broke so we wired a bright red push button directly from the battery (well - actually a fused 12v line inside the car) to the starter motor coil - it has since worked like a champ for nearly two years.
First off - DO NOT wire directly from the battery, there is always a potential for a short (say in an accident or due to your own mistake) and those lead-acid batteries can be unpleasant when they explode. Nothing will ruin your day than having a car fire after your accident, use an inline fuse at least, using the cars existing fuse/breaker system would be better (i.e. use an existing 12v line)
Your best bet would be to splice the 12v from the accessory 12v line in the harness (it should be exposed near your radio) Since this is fused and isolated from any more important components. TYPICALLY the horn is on its own branch, just because it needs to be activated by the alarm system and should be able to honk when the key is out (the accessory 12v is usually shut off when the key is taken out), however this may not be important to you.
The proper way would be to look up the wiring diagram in your car and find which wire in the harness corresponds to the horn 12v supply (it may be tied to some other 12v line - like internal illumination) and which wire corresponds to the horn, -tie the two together with a switch and you are good to go.
The "stupid college student way" (I only say this because this is what we did with my friends ignition) to approach the problem would be to pull a wire through the firewall and directly to the horn, tied to some random 12v line near the dash.
As for placing the switch... I would avoid opening up the steering wheel, but thats just my squeemishness around airbag systems, although I don't know if your car has a driver airbag so it may be clear. And depending on the rules in texas, you may be required to attach it to the steering wheel somehow.