You can fit a single horn without the relay. It's still an improvement. Two are a big improvement to standard without bursting your eardrums, but loud enough to get you noticed - a nice fat twin 'air horn' sound.
Discrete fitting is straight forward - one screws directly to the bottom threaded bolt hole for the standard horn bracket (gets warm down there but horn polypropylene body hasn't distorted). You'll need to drop the battery box slightly so you can spin the horn on without it hitting the exhaust. The second, including the relay, bolts neatly into the gap above the Oil Tank filler cap sufficient to still get access to remove the filler cap etc. You'll need to drill a hole in one of the Stebel brackets provided so you can bolt it to the top fitting of the coolant bottle. Take particular care to mount it away from the rad fins!! With the tank & belly panels back on, you can't see them.
Wiring takes time to hide away properly as you can't get a fag paper in anywhere - otherwise it's a straight forward mod.
The Stebel wiring diagram is wrong. Here's one I did earlier:
