Shut off the home's main water supply valve and turn off the circuit breaker switch that controls the electrical power for your furnace. Drain any standing water from the pipes by opening a faucet or flushing a toilet.
Choose a location for the humidifier on the furnace's cold-air return ductwork. Tape the template from your Aprilaire 550 humidifier's installation hardware package onto the duct at your desired location. Use a level to make sure the template is straight. Trace around the outline and mark the starter hole and screw holes on the ductwork, then remove the template.
Use an awl or the end of a Phillips screwdriver to make an indentation at each marked hole location. This indentation will keep the electric drill bit from sliding around on the slick surface of the ductwork.
Drill the screw holes and starter hole at each awl indentation. Insert the end of a pair of tin snips into the starter hole. Cut around the marked outline to create an opening for the humidifier. Line up the opening in the rear of the humidifier with the hole you cut in the ductwork. Use the mounting screws included in your hardware package to secure the humidifier to the duct.
Create a water line for the humidifier by connecting a saddle valve to the home's existing cold water supply line. Run copper pipe between the open end of the saddle valve and the water inlet valve on the Aprilaire 550 humidifier. Rotate the saddle valve's handle clockwise to make a hole for the cold water to flow into the copper pipe. Attach the catch basin to the bottom of the humidifier.
Install the humidistat on the return ductwork at least 12 inches above the humidifier. Repeat steps 2 through 4 to trace the installation template and make the required holes for the humidistat. Mount the humidistat on the ductwork and tighten the screws. Connect the electrical wires to the humidistat and furnace power supply.
Open the home's water supply valve and turn the circuit breaker switch back on. Inspect the connection points between the water lines and humidifier to make sure no leaks exist.