The improper way should work. Not all that improper to be honest. Cutting the ground has always been a reliable tried and true method.To do it properly and cheaply, Multiple switches.
Properly (and slightly more expensive), 1 switch and multiple relays.
cheap and improper, 1 switch, use it to open circuit the ground on all of the lights.
(no idea if this will work, but just taking a guess, maybe ill try when im home tonight)
To do it the improper way... Under the seat, there should be 4 green wires grounded to the frame (right behind the tank). Determine which of these wires is the ground for all the lights, horn, and kickstand switch (you may need to bypass the kickstand switch). Your switch should go between that wire and where it used to be.