Turn off the breaker switch that controls the furnace power. Shut off the water at the home's main supply valve. Open a faucet to drain the remaining water from the system.
Open the humidifier's packaging and take out the installation template. Tape the template onto the ductwork inside your furnace's cold-air return. Check the bottom edge of the template with a level. If the humidifier is not installed in a straight line, water may leak out of the reservoir.
Trace around the template, marking the locations of the mounting screws and the starter holes for the humidifier's opening. Remove the template and make a small indentation with an awl or screwdriver at each hole. Use an electric drill to finish making the holes.
Insert the end of your tin snips into one of the starter holes you drilled in the previous step. Use the tin snips to cut out the opening for the humidifier according to the installation instructions.
Hold the humidifier up to the prepared ductwork. Insert and tighten the mounting screws to secure the humidifier.
Attach a saddle valve to your home's cold-water supply line. Connect a piece of copper pipe to the open end of the saddle valve. Connect the other end of the copper pipe to the humidifier's water-inlet valve. Turn the handle of the saddle valve clockwise to pierce the cold water line so water can flow into the new connection.
Follow the installation instructions in your owner's manual to connect the overflow drain or catch basin, whichever applies to your humidifier. if your humidifier includes a humidistat, attach it to the plenum near the furnace's thermostat.
Turn on the breaker switch and open the main water-supply valve. Check the humidifier's plumbing connections for leaks before testing the unit.