You now start the car with the AC system on high. You open the can tap so freon starts to travel from the can to the suction side. You only need to open the can tap 3 to 4 turns and you will feel the top of the tool getting cooler. This tells you the can is open enough. Don't try to unscrew it all the way to make it go faster as the needle can be screwed out of the can tool and then you have a fountain of freeze spraying all over- not good. Freon moves from the can to the suction side because the can is under low pressure but the suction side is under even lower pressure so the freon leaves the can and moves to the system. Now watch the accumulator can. If you can safely touch it you will see it's warm if your low on freon. As you charge the system it will become cooler. Finally it will start to sweat. That is when you stopAdding freon. Now you have about the right amount. If you add too much your over stressing the system and it is not going to work any better in fact it will not be as efficient. Overcharging also adds stress to the system since pressures are higher than normal. So now that the accumulator is sweating close the can tap. Check the temp in the car and you should feel very cool air . I dropped from 87 to 50 degrees with a can and 1/2 of another. The cans come in preset quantities. I don't ever buy the large completely charge your system can as it is usually too much. The smaller cans are what you need. I don't recommend the sealer in a can. If you think you need to add oil to your system take it to a shop so its done correctly. Any left over freon do not vent to the atmosphere. tighten down the can tap and leave the tool on the can. You can tell when a can is empty as if you shake it it will feel empty.
Close the tap before you remove the hose off of the fitting. Then recap the fitting. If your system does not work properly ( no cold air ) it is time to go to a pro. Open freon can be used at a later date but must be stored in a cool place That is CHILDPROOF. The temperature coming out of your vent can vary depending on a lot of factors so don't think you have to see the temperatures I saw.