How do you add freon to the air conditioner system in the Pontiac 1992?

Freon (R-12) is way too expensive as it is no longer in production. Have it converted to use R-134a. I suggest a visit to a professional A/C shop as the fact that you need to add refrigerant indicates you have a leak that needs repairing. It is illegal to add refrigerant to a system with a leak without first repairing the leak.