To open the switch box, disconnect the power. Do this by disconnecting the circuit breaker. Take the screws off from the cover plate. To be safe, check the power using a voltage tester.
Double-dimmer switches come in a variety of styles. For example, you can choose to purchase one with knobs or a touch sensitive one. The capacity of the switch is also something to consider. This is measured in watts. Calculate the capacity for all the lights you want the switch to control.
Note down the height, width and depth of the box. Next, count all the wires, and calculate the volume needed.
Use a hacksaw blade to remove the nails. Next, take off the screws from the box clamp. Now you can remove the box and all the wires.
With a sharp pencil, mark the spot on the wall where you want to place the box. Do this by tracing the profile of the box. The next step is to cut the hole in the drywall. Follow the pencil mark. Place the wires in the box, and attach it to the wall.