Depending on the horn relay you have the terminals may be marked. 1 will be battery +, one will be load (horn), and the other will be ground (horn button). The horn button gounds the horn relay and the relay sends battery voltage to the horn. The relay is nothing more than an electric controlled switch. The horn switch is a manuel controlled switch. The manuel switch (horn button) tells the relay what to do.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"