Recharging a car's air conditioner is best left to a licensed professional. If you are going to do it yourself, you can only use R134a refrigerant unless your car was made earlier than 1995. In those cases, have a licensed mechanic retrofit that car's system to take R134a, because the older R12 refrigerant it uses is very rare and expensive. Also, wear protective glasses while charging the system and don't get any of the refrigerant on your skin. Make sure only the can's vapor goes into the system, not the liquid.
Connecting the Canister
Locate the service fittings for the car's air conditioner. There will be a low side fitting around the suction hose line that runs from the accumulator to the compressor; the high side fitting is on the line going from the compressor to the condenser. Connect a recharge service hose and valve to the refrigerant can. First, turn the valve on so it punctures the top of the can, slowly turn it back out until there is a little bit of refrigerant vapor in the hose that has helped blow the air out. Once the refrigerant is in the hose, close the valve to keep from losing refrigerant. Once the valve is closed, you should immediately connect the hose to the low side fitting on the AC system. The high side fitting is too dangerous to connect to because of high pressure; it is likely designed to not fit the hose as a precaution. With the can and hose connected, hold the can upright to prevent the refrigerant liquid from getting in the hose. Now, attach a high pressure gauge to the high side fitting.
Charging the System
With the refrigerant canister and pressure gauge connected, you now need another person to turn on the car from inside and turn the air conditioner up to the highest setting. When you open the valve on the service hose, the refrigerant vapor will flow into the AC system. The person inside the car should now feel the air from the vents getting cooler. Watch the reading on the high pressure gauge as the system charges. When the reading is between 225 and 250 PSI (pounds per square inch), close the valve on the hose. Make sure it's completely closed before you disconnect the hose from the low fitting, then disconnect the gauge from the high fitting. Replace the caps on both fittings.
