My '94 GT is R-134a.
In order to properly recharge an AC system, you really need 3 things, cans of refrigerant, gauge cluster, and a vacuum pump.
1. With your car off, hook up your gauge cluster. On one side of the gauge cluster there should be an extra fitting and the gauge may even read vacuum too. Hook up your vacuum pump there and let it run for 1/2 hour or so (grab a sandwich, beer, "quickie", or whatever)
2. Close valve to vacuum pump, then shut off the pump and remove it.
3. Start car, put AC on max (don't worry about killing your compressor, the pressure switch won't turn it on until there is refrigerant in the system) attach a can of refregerant to the low pressure side of the gauge cluster (you may want to purge some of the refrigerant to clear any air out of the lines). Then open the valve so that the refrigerant gets sucked into the system. As you fill your system, the refrigerant will take longer to get sucked up. You will probably have to warm the can with your hands (heat of vaporization) and shake it alot.
4. The number of cans of refrigerant you buy will be based on the volume of your AC system (see your manual). There should be pressure ranges that you will want to fill to based on your manual as well.
5. Close valves on gauge cluster, remove fittings, and give it a test drive and enjoy your cooler car.