Get this schematic from below and have it in front of you so we could connect everything as in the schematic. The heater to the relay and the relay to the main 220V input with a mains wire. The same input is connected to the 5V power supply. The Arduino is connected to the LCD, to the buttons and the rotary encoder. Is also connected to the control pin of the relay. The thermocouple is connected to the MAX6675 module and the module is connected back to the Arduino. On the back I make connections to the power cord for earth and live input and that’s it.