Remember to do this when the system is operating. You will notice the compressor kick on when the pressure switch senses enough psi to turn on the system. Also, when operating it will "suck" the freon into the system fairly quickly. Turn the can upside down so the liquid is pulled into the system, not upright where it would siphon in the gas component (That takes forever.)
If the system has lost
freon, there's a reason. I would also think about a dye check too so you can fix the issue and not keep wasting time and $$ putting a can in every so often.
If the system has gone to atmospheric pressure (e.g. no freon in at all) then you really need a vacuum pump to pull all (or at least down to 20-30"Hg) of the 'wet' air in there out before introducing the freon. You'll get a better operating system that's got more freon in it and won't ice up and also won't kill the drier prematurely.
Watch the sight glass and when the bubbles disappear in the fluid stream you got it. Or use the gauges to determine the proper amount of pressure ont he high and low side.. read up on the refrig process online. It's really simple.
In reply to the fridge question: Refrigerators are usually serviced using a solder sealed method. Most handi-types like us boat people don't have the gear
to do that kind of service
. Usually it isn't needed. I've never worked with a saddle clamp concept
of re-furbishing a frig but if I had the chance I'd have a pro do the job and have them solder in a fill port so i could do in-field repairs
to the filling part of the job.