A/C Pro is a refrigerant kit for DIY car AC recharge. First, start the car and set the AC to maximum cooling. Then, locate the car's low-side AC service port and connect the A/C Pro canister's recharge hose to it. After the gauge shows the current pressure, you can check the ambient temperature and compare it with the gauge reading. If the system is low, you can start adding refrigerant. Always follow the product instructions for safe and effective use.