1. locate a new switch in new or existing box.
2. bring 'hot' black and white wires to this new switch box (breaker off of course) from nearest source.
3. run new wiring (black and white wires) from switch box to disposal, behind wall.
4. remove screw(s) holding electrical connection cap on disposal.
5. strip ends of black and white wires and install on disposal screw posts
6. make connections at switch box: white to white in box with a screw cap. push to rear of box. use electrical tape over cap if you want to. one black (hot feed) to either screw on switch. other black (from disposal) to other screw on switch. a single pole switch has only the 2 screw posts. by throwing the switch you interrupt or complete the circuit over the black wiring.