Air Conditioning Charging Station

How do I add Freon to my 2000 Dodge Intrepid?
My air conditioning does not work on sector. In the past, the problem was solved by the addition of freon. The Mobil service station near intends to charge $ 200.00 for this work to be done. Maybe you can help us indicating how this can be done.
Gotta some A / C gauges. Kinda need to know what you do. Gotta be certified by the state.
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