Turn off the power at the service panel. Check the wires with the circuit tester to be sure the power is off.
Orient the receptacle box horizontally against the wall in the desired spot. Trace its outline with the felt pen and cut out the outline with the keyhole saw.
Punch out the holes in the box with a screwdriver and insert the electrical cable into the box. Since this is a horizontal installation, removing the knockouts on the side of the box will allow you to attach the wires without having to feed them around the receptacle
Insert the box into the hole. Attach the box to the wall by turning the screws on the clips or flanges.
Attach the electrical wires to the receptacle by connecting the hot wire to the brass screw, the white wire to the silver screw and the bare ground wire to the green screw.
Insert the receptacle into the box and attach it with the screws. Attach the cover plate. Turn on the power and test with the circuit tester.