To install your smart thermostat, start by turning your HVAC system off. If you had an old thermostat, remove its faceplate. Using your smartphone, capture an image of the wiring. Plug the smart thermostat into the HVAC system using the wires in the backplate. If the HVAC system has no C-wire, install it. Position the backplate of the new thermostat on the wall, mark, and drill screw holes. Connect thermostat wires to corresponding terminals. After installing the new faceplate, connect the smart thermostat to your home Wi-Fi network. Finally, link it to the app on your phone or tablet. If you are not replacing an existing thermostat, you may need to get a professional to hook up all the necessary wiring to your system.