In the original mod Philip Ruetz snipped one of the resistor leads, which permanently removed the resistor and a capacitor from the circuit. Adding a switch simply allows you to reconnect the resistor and "undo" the mod. The relevant resistor in the Mouse is R5, which is the 47 ohm one about halfway up the board, next to the 560 ohm resistor.
I recently built a Mouse with the mod using a SPDT On/On mini toggle switch, which is easy to get hold of. To do this version of the mod solder the resistor to one of the eyelets so that it stands up on end and solder a wire to the other end of the resistor. Solder another wire to the other eyelet in the PCB. The switch has 3 pins; solder one wire to the middle pin and the other to either if the other pins. There are pics of my Mouse which show how I located the switch here: viewtopic.php?f=8&t=34068&p=283629&hilit=fledermaus#p283629
I would recommend a slightly different approach, which is easier and less fragile: use a DPDT On/On switch, which has 6 pins. Connect each of the 2 resistor eyelets to one of the middle pair of pins and solder the resistor across either of the other pairs of pins. Job done.
I'd definitely end up gluing my hand to my crotch if I tried that.