Ok, for the horns here is what I would do. You said one horn works, right. Un-plug both horns, use the multimeter to test for voltage at the horn connectors.
The multimeter should have DCV settings. Likely it has a 20 DCV, use this setting. It might have different places to plug in the test leads, so make sure they are correct.
I assume it is a digital multimeter so don't worry about the polarity. Connect the test leads to the horn leads. Note there might a short ground
(-) wire that connects the negative terminal on the horn the chassis and only one wire that leads back into the bike. Press the horn button, you should be getting 10-13 volts.
Test both leads. If they both have voltage then likely the horn is bad.
Another quick is to just swap the horns. If the one that does not work starts working then you have a wiring issue.