You plug the thermostat into the wall, then plug the heater into the thermostat. A higher-end thermostat will have a probe and a digital display that shows the set temperature and the actual temperature as recorded by the probe. If yours lacks these features and simply has a dial on it, you need a digital thermometer with a probe to calibrate the thermostat. And once it's calibrated, you might have to adjust it as the temperatures outside the cage change.