Stan's reposting of Dave Stragand's article covers just about
everything. The only thing I'd add is...After the conversion, instead
of using R134, use HC-12a, or similar propane derivative freons. These
are sold under various names, such as DuraCool, Freeze 12, etc.

They give as good a cooling effect as the older R12, but have a much
lesser head pressure (and hence on our Vanagons the compressor has to
work less hard).

In the US, to legally use them you must first convert the vehicle to
R134 oils and fittings and then you must properly label your vehicle.

I use them in the GoldBrick and my Westy, and I'm very happy about them.