Locate the place where the receptacle is going. Use a stud finder to make sure there are no studs in the way.
Place the receptacle against the wall and lightly mark around it with a pencil.
Drill a 1/4-inch hole in the upper right corner of the marked area.
Insert the tip of a keyhole saw into the hole and saw downwards. Follow the line drawing around until the piece of drywall comes loose.
Insert the receptacle into the hole to check the fit. If it is too tight, sand the sides of the hole with coarse sandpaper until it fits.