| Take the positive lead from the jack socket (tip) unsolder this, then solder it to one side of what looks to be a SPST switch. Solder a wire from the other side to to the jack socket.
That would be the simplest way as far as I can see, leaving the connection to ground should keep things safe and hum free but I can't guarantee that as I don't know if noise would be introduced from the switch itself. It may hum or click and someone else might have a better place to break the circuit.
Also those switches are cheap and tiny, 2 seconds too long with a soldering iron and they start to melt apart, I'd personally have picked a switch with a 'chunkier' throw to prevent accidental switching. | 
