In order to join two or more gangable boxes together, you must first remove one set screw from each of the boxes that hold on the side plates. Use a screwdriver to remove the screws and set them aside for the reconnection process. They have right-handed threads, so you’ll be turning the screwdriver counterclockwise.
Now that the setscrews have been removed, pull and twist the side plate until it comes free from each of the two boxes. You’ll notice that the other side of the plate has a groove that the side plate notch fits in, which holds it securely. Completely remove the plates and discard if you’d like. These will not be needed in the installation process.
Slide the two boxes together, open side facing one another, and replace the setscrews that you removed earlier. Using a screwdriver, tighten them by turning them clockwise until firmly secure. The boxes should be lined up and appear to be one double box when the screws are secure. The box is now ready to be cut into the wall and have wire added to it. It’s just that easy and almost anyone can do it!