The most common rotary switch that you would probably want to use would require that you find a place in close proximity to the light fixture which would allow you to drill a hole in which to install the switch. The switch would have a threaded shaft around the rotary knob which you would install in the drilled hole and a nut (possibly with a washer) with which you would secure the switch in place. After you have the switch secured, located the black wire connected to the light. When you locate the wire you may find that the incoming power lead and the lead to the light are connected with a wire nut. (The length of the conductors may dictate where you install the switch) If there are two wires connected with a wire nut, remove the wire nut, clean up the ends of the conductors, and connect to the switch.
If you find that there no place close enough to allow both wires to connect to the switch, locate the switch close enough to one of the wires to connect it to the switch, and add a length of wire (same size, rating, and color) to connect and extend the short wire to the switch.
By ALL MEANS - Be sure to unplug the power cord to the machine before you begin!
After all wires are connected (I also recommend using electrical tape to ensure that wire nuts are held in place and to remove the possibility of any bare wire near the ends are covered) plug the machine back into the outlet and test the switch.
If you find that there no place close enough to allow both wires to connect to the switch, locate the switch close enough to one of the wires to connect it to the switch, and add a length of wire (same size, rating, and color) to connect and extend the short wire to the switch.
By ALL MEANS - Be sure to unplug the power cord to the machine before you begin!
After all wires are connected (I also recommend using electrical tape to ensure that wire nuts are held in place and to remove the possibility of any bare wire near the ends are covered) plug the machine back into the outlet and test the switch.