Switch the rotary switch to 20 in the DCV section....black test lead goes on the COM socket and red test lead in the V ohm,mA socket. Put the black lead to the bike frame or engine ground connection and then put the red lead to the terminals on the horn....with the ignition on you should have 12v ish on both terminals....then when you push the horn button one of the terminals should go to 0v and the horn should sound. The horn has a 12v supply going to it and the horn button connects the other side to ground, putting 12v across the horn. Be careful though, coz the horn on my old bike didn't sound unless the engine was running.....so just be wary of that.