Wire from battery (+) through the ammeter to horn, wire from horn (-) to horn button. When the horn button is pressed it contacts ground and completes the circuit - horn blows.
This if for a '38 but yours will be similar. This diagram is for the stock single horn. The accessory horns will need a relay.
Edit, after looking at your picture again I think I see the problem. Someone has mounted half of an accessory horn set as a single horn. The accessory horns have only one wire, a hot wire. They ground through the housing. You have a cobbled together horn set up for sure. If you want to keep the same set-up you'll need to run a hot wire to an accessory two terminal button/switch of some kind then from the other terminal on that button/switch to the horn. I'd strongly recommend you put a fuse in the circuit.
This is what the accessory horns normally look like. Edit #2, an alternative would be to isolate your horn's housing from ground, connect the hot wire from the horn to the battery (fuse in the line), then run a ground wire from the horn's housing to a one terminal horn button/switch. This would likely be the easiest and safest if you want to keep that horn.
