To charge a car's air conditioning system, check the ambient temperature, attach the coolant container, turn the car on, read the pressure gauge and fill the system until it reaches the appropriate pressure. The process may vary depending on the brand of recharge kit, but it should be similar for most vehicles.
Use a thermometer to make sure that the ambient temperature is within the operating range of your recharge kit. Locate the low-pressure port on the vehicle's air conditioning system, remove its cap and make sure the port is clean. Attach the hose from the pressure gauge included in the charging kit, and then attach the refrigerant container to the pressure gauge. Set the gauge to the ambient temperature. Start the vehicle, and observe the air conditioner compressor. When the compressor engages and begins spinning, note the pressure on the gauge.
Pull the trigger on the coolant can, and begin charging the system until the desired pressure is reached. The pressure gauge is only accurate when the compressor is cycling. Remove the hose from the low-pressure point, and screw the cap securely onto the port. Make sure the cap is sealed and there is no coolant leaking from the port. If there is still coolant left, leave the gauge attached, and store the kit in a cool dry place.