Hi Alan,
As Ian says - what I think they are showing you how to modify an existing horn circuit, using the top switch to give the option of original horn or air horns and the other switch to disable the air horns.
If you already have (or had) a working air horn set up on your car I would think you could simply replace your old compressor & horns with the new one. You should then only need to take the wires that went to the old compressor and connect them to your new one.
Good luck, Steve