To charge the refrigerant in a window air conditioner, first attach pressure gauges to the low and high sides of the system to check current levels. If the refrigerant is low, connect a refrigerant cylinder using appropriate valves and hoses. Slowly add refrigerant while monitoring the pressure readings to ensure the correct amount is added without overcharging. Use proper safety precautions and protective gear when handling refrigerants. Finally, verify that the unit is cooling efficiently after charging.