Wiring A Second Horn Into A 9N With Relay And Switch

Hi

I'm looking at getting a second horn, an 'ahoogah' one, to sound at the push of a button. I'll have the button on one of the blanking plates by the hazard switch. The horn will only sound when ignition is on. From what I gather, the horn circuit is:

+ battery terminal -> fuse -> relay -> car horn -> - battery

and the switch:

accessories + supply -> fuse -> switch/button -> relay (as controlling circuit) -> - battery

problem is, I don't know what fuses, wire, or relay I should use, and I'm not sure how to go about installing extra wires in the car, what wiring tubes lead where, and so on. Any guidance would be appreciated!