Mark your copper pipe at the length you wish to cut it.
Turn the thumb-screw counterclockwise until the tubing cutter is open far enough to fit over your copper pipe.
Slide the tubing cutter over the copper pipe and turn the thumb-screw clockwise until the cutting wheel is tight on the pipe.
Rotate the tubing cutter around the copper pipe while holding a firm grip on the pipe with the other hand.
Tighten the thumb-screw 1/4 to 1/2 turn after rotating the cutter around the pipe several times. Continue to rotate the cutter around the pipe and tighten the thumb-screw until the pipe is cut.