You can add freon or refrigerant by using a car AC refrigerant kit, which usually costs around $35. The process entails attaching it to the car's low-pressure service port, setting the gauge according to the ambient temperature, and then refilling the system with the engine running until the gauge reaches the appropriate pressure. However, this isn't recommended as it's not accurate and can cause damage in the long run. It's better to have a professional do it for you with the correct tools, which usually cost no more than $150.