What's the proper procedure for adding R12 refrigerant?

Well both CAN be right--just depends on the situation and your experience.

The DANGER is that with the can upside down you are feeding in LIQUID, and piston pumps do NOT like liquid in the cylinders and valves. So if you feed in too much too fast you can "slug" the compressor. You might get to hear what a breaking rod sounds like.

The SAFE (if slow) way is to charge gas (vapor) with the can upright. There is almost no way to slug a compressor with liquid by doing this.

Frankly, unless you have a little experience, have read the "book" and know how to use and interpret gauges and thermometers, you might just overcharge the system.

Under certain conditions, this can result in dangerous (for the compressor) high head pressures, liquid returning back to the compressor (more slugging) and a few other problems.

Don't feel bad, I used to maintain commercial and home refrigeration and AC/ heating, and I've found a few units operating dangerously overchaged, as well as undercharged --and it turned out there was no leak, either.

Undercharging is also not good, although usually you'll notice it as reduced performance. The bad for the compressor, is undercharging does not get ENOUGH cool refrigerant back to the compressor, and the valves and head can run too hot.