the key to easy filling is to upend the source tank

Yup, buy the attachment that others are showing you. I make some very soapy water and put one drop on the relief valve and work in back and fourth. That's just to lube it (you never want to use any type of oil or oil based product) Then freeze the 1 pound can for a few hours. It helps if the large 20 or 30 pound tank is a little warm so set it in the sun for awhile. Screw in the attachment and get the 1 pound can out of the freezer. I put 1 drop or soapy water on the filling valve screw it onto the large tank, flip the tank over and open the valve. You will hear it fill. when it done, turn off the valve, flip it back over and unscrew the 1 pound tank. DONE I always check for bubbles on relief and fill valves (with soapy water) and I never store a refilled tank in the house. Store them either outside or in a drafty shed. They can leak. Better yet, only fill what you are going to use in the very near future.

Trying to understand why everyone freezes their 1lb containers? I've never done this and they fill up way fuller than what you get at the store. Just turn the big tank upside down and in half an hour the little tank is as full as it can get.

We aren't waiting a half hour to fill a bottle. By freezing the small bottle, you are reducing the pressure so when you hook up the big bottle you will get a better fill as the pressure between the two tanks equalizes. I only "fill" a bottle for about a minute. Oftentimes, I will be filling a dozen bottles at a time and at a half hour a bottle, I wouldn't have time to make it fishing the same day!