I know what you are thinking, why build this when you can buy a refill adapter ready made for your tank. I used to use one, but you can only fill your bottles about 2/3rds full. This method allows you to completely fill a bottle. much the same way as they were originally filled in the factory. and with the quarter turn valve, the refill process is quick and easy.
Disclaimer: this is the way I refill cylinders. I do not claim to be an expert, and I take no liability for mistakes you make. BE SURE TO DO THIS OUTSIDE preferably with a bit of a breeze and no near, flames, sparks, cinders, etc. Propane is highly flammable and this can be dangerous if you don't use common sense. If you blow yourself up, don't blame me!
Step 1: The Anatomy of a 1lb Cylinder
Step 2: The Valve
Step 3: The Vent Tool
I no longer use the tire valve tool to vent the small tank. It is safer to use a needle-nose pliers to pull up the vent valve. The pressure relief valve is set at the factory. (as mentioned in some comments below.) it is difficult to get this back into the same setting and can lead to preventing the safety valve from working. the following is a link that I have since found that demonstrates the refill process with needle-nose pliers. (And yes it can be accomplished with a Harbor Freight valve adapter, but it is much easier to use an angled setup with more room.)
Step 6: Completely Fill the Cylinder!
When I am done, I cover the top of the 1lb cylinder with a bit of foil or plastic wrap to keep the connector clean and to mark the cylinder as full. If you save the plastic connector cap that came with the bottle, that is even better. IF you need more bottles, check the dumpster at the camp ground, there is always a plethera of them discarded by people who do not reuse...
You can now save money, and keep the cylinders out of the land fill. Keep your cylinders dry and clean. replace the cylinder if it gets rusty or starts to leak. When in doubt, error on the side of caution.