Every product will have slightly different instructions, but most of them fall within certain parameters. Your air conditioning system has two valves. One of them is a high-pressure valve, and the other is a low-pressure valve. Often, the caps of these valves will be marked with an “H” or an “L.”
With most products, you just hook the can up to the low-pressure valve and release it. A gauge is used to determine how much is needed, and thus prevent overfilling. Make sure you wear gloves and eye protection when doing this, as the refrigerant can be pretty hard on the skin and even harder on the eyes.