Mark a straight line along the wall a few inches from the floor just below the top of the baseboard. Cut the wallboard along this line using a drywall saw for the hollow parts between the studs and a drywall knife to cut over the studs.
In the center of each plate notch, cut a second, 3/4 inch notch for the cable. Make the depth about 1/2 inch to accept the cable thickness, but keep the entire notch depth to a maximum of 7/8 inch for load-bearing 2x4 studs and 1-7/16 inches for non load-bearing studs.
When tapping an existing outlet box it will always be OK to extend the circuit from a box with fewer than the maximum wires running to it, as long as the total electrical draw from the other outlets in the circuit don’t exceed the ampere rating. To determine the load of the circuit, use the chart and formula for calculating existing load to extend a circuit at this link.
A wall outlet box that houses two devices side by side as with two wall switches at a door. Triple-gang boxes are also commonly available to hold three devices in one place.