How do you add freon to a 1992 GMC Vandura?

If you are qualified to service the A/C system, then you know how. If you are not qualified, then you should take it to a local service shop for repairs. Refrigerant can be dangerous if mishandled.

You can also now buy recharge kits at Canadian tire and do it yourself, the refrigerant is now ozone free so it's no longer dangerous. Just follow the instructions on the kit, it's pretty easy