I guess from your reply you are not electrically minded, so just for your peace of mind I'll expand slightrly on Nos' reply....
The kill cord switch replaces your old stop button. Inside the cowl wil lbe a nut or something to ld it in. Remove the entire switch assembly and replacw with the kill switch.
How it works:
The switch basically earths the supply to the spark plug coils. Electricity will always take the easiest route to earth, so your stop button does exactly that - provide it with a nice resistance free route. (easier than going round the coils). No power to coil = no power to spark plugs = no spark = no go.
The kill switch does excatly the same thing, the only difference being that the contacts inside the kill switch are sprung shut (and held open by the kill cord clip) whereas your current stop switch is sprung open, and you close the contacts by pushing on it. To stop the engine with the kill switch just pull the cord off.
Hope that makes it all a bit clearer.