The valves on the hoses near the couplings open to allow the gas to get to the gauges. The valves on the body allow the refrigerant from the can to be directed to the hose that you want to add the gas too, usually the low side. I think the valve for the high side is if you have a vacuum pump hooked up to suck the old gas out for maintenance.
On the valve for the can, first screw the can to the valve, then screw in the needle.
I’m trying to figure out “good” too.