Ok basically, before you want to do any modifications to your guitar, I would suggest you familiarize yourself with the wiring inside a guitar. Now onto the kill switch. Basically you need a SPDT switch.
Then, the output wire (the wire that is connected to the output lug of the jack socket) gets soldered to the middle leg of the switch.
One of the outer switch legs then gets soldered to where you just disconnected the output wire from. The other switch leg then gets soldered to ground. This way when the switch is in the "kill" position, it is grounding the guitar signal. When the switch is not in the "kill" position, the signal just goes to the output jack as normal.
Hope that all makes sense.