Like someone else said
DON'T remove your bumper. It will make it easier to get to the horns, but it's a lot of effort you don't have to go through. The stock horn is a Fiamm trumpet style horn and is mounted on the driver's side behind and slightly towards midline of the fog light.
Most people who have done horn modifications have done one of three things. 1) added a low tone horn to the existing OEM high tone. 2) replaced the stock hi tone and added a low tone with an OEM style trumpet horn. 3) replaced the OEM horn with non-trumpet (disc style) horns placed in the grill.
I replaced my stock Fiamm hi tone with hi/lo combo Fiamm freeway blaster horns. Each is 138 decibles, and sounds wonderful. I put these in the stock location. It's a little tight in there but it can be done.
Here's a thread with info on the horns, and links to lots of discussion about this mod. There's also a sound clip of the freeway blasters in that link.
If you want to do this mod with stock style horns in the stock location this is what you do.
DISCLAIMER: The following is for informational purposes only. Any modifications to your car are taken at your own risk, and I am not liable for any damages/injuries to your vehicle, or yourself while you are performing this modification.
-park the car on a flat surface
-raise the front of the car (jack stands, ramps, vehicle lift). I don't suggest lying under the car on only the stock jack. It much easier to get to everything if you can fit your face and both hands under the bumper.
-remove/loosen the plastic baffle on the driver's side of the bumper by removing the 3 (?) screws on the bottom, and removing the nylon screw/fasteners on the front side of the wheel well. It helps to have the wheel turned to get to those.
-look up into the space where the fog light is. The stock horn is attached to a bracket that is mounted to the frame of the car. Remove the single bolt holding it to the frame, unplug the hot wire from the horn, and pull the horn/bracket out of the car.
-if you're replacing the stock horn then remove it and add the new one in it's place. If you're simply adding a low tone horn then attach the new horn to it's provided bracket, and attach the new horn bracket to the stock bracket on the bolt from the first horn. It might take a few times of trying to put the horns in/out to figure out the correct angle that the brackets need to be connected at. Make sure both trumpet bells are aimed downwards, otherwise they will fill with water and stop working.
-make sure you attach the ground connector of the horns to the brackets via the included wires (the stock horn only has one connector).
-make your new wire connectors. Use about 1' of wire in two 6" sections to give yourself plenty of room to work with. Strip both ends of the wires, and wrap one end of both wires together to make a "V". Place a single male spade connector on the connected wires and crimp/solder in place. Place a female spade connector at the end of each remaining wire.
-connect the female ends of the wires to the remaining free connector on the horns.
-replace the combined horns back into the car, and attach to the frame with the original large bolt, and plug the male end of the wires to the stock wiring.
-use your horn and check to see if things work.
This is what the horn brackets should look like together with the wires.
If you want I can probably dig up more pictures, or answer any questions you might have.