A car horn relay is nothing more than a "remote light switch" from an electrical standpoint. The relay is normally open with power from the battery to the relay and from the relay to the horn. The horn is grounded to the frame. The other two terminals are used for the remote activation. The second power terminal can share the power from the primary input terminal. The secondary ground terminal travels to the horn button. The horn button grounds the relay allowing power to flow through the activation circuits which in turn close the primary circuits allowing power to flow through the horn and to its ground on the frame.
Locate a place for the relay on or close to the radiator support shroud. Bolt the relay solidly to its intended location.
Connect a wire from the battery to the battery+ terminal of the relay, using the appropriate terminal for the battery and a spade terminal for the relay. Connect a wire with a spade terminal to the horn terminal on the relay and run the wire to the horn battery+ terminal using a spade terminal for the connection.
Connect the horn ground to a suitable ground on the frame.
Connect another wire to the ignition+ terminal of the relay from either the battery or ignition switch accessory terminal. If the wire is connected to the battery the horn will work with the ignition off. If attached to the ignition accessory circuit it will work only when the ignition key is on. For a battery connection, it is better to cheat and install a short wire from the battery+ relay end of the wire to the ignition+ terminal. Just put both wires into the battery+ spade terminal.
Connect the wire from the horn to the ground-side of the relay.