You need to find the correct wire that will be the hot wire for the horn.  It might take a little testing with a test light that has the metal probe on the end to find which wire is hot when the horn is blowing.  That is if you can not see which wire goes to the horn. 

Once you have the horn wire discovered you need to simply hook  a wire between the signal light wire and the horn wire.  Auto shops have what I call a clamp on to splice a wire into the existing wire.  I don't usually use them because after time they loose their connection and then you no longer have a serviceable wiring job. 

So any way you get a proper length of wire and to of these clamp on connectors and simply wire the horn wire to the turn signal wire.  Every time the blinker blinks the horn honks. 

You can hook up the horn wire to the brake light wire and every time they step on the brake the horn will sound.