Real simple, just don't smoke around the tanks.
Wear gloves and goggles.
No need for a scale to weigh anything.
No pumping required so long as the tank you're filling has an 80% fill level port (thumbscrew or screwdriver, keep reading).
The bulk tank has to have a liquid valve.
Use propane rated hose.
Install a ball valve as a secondary valve just off the tank liquid valve.
If you're not intimately familar with pressurized vessels and liquified gasses you'll need your dealer to install the liquid valve on the bulk tank. Not every tank has a liquid outlet fitting. You may need to change tanks or when the tank is empty go with a hybrid (and somewhat uncommon) 3-way valve.
If your tank has a fitting labeled 'liquid', sticks up an inch or an inch-and-an-half, looks like a hex fitting, and has a plug in it, you most likely have the liquid outlet available for use. In theory the plug can be removed, you'd get a spurt of liquid propane and then the spring loaded excess flow fitting would engage and the flow would snap stop. Then the propane-tank type approved liquid valve could be installed (with a quality ball valve after it before the hose is attached). Occasionally the excess flow fitting doesn't shut. That's why you let your service man do this unless you are familiar with the process.
The liquid line off the bulk tank will probably have a 1-in or 1-1/2-in fitting on the other end. Use an adapter to fit to your 20-lb propane bottle. Make sure your adapter will open the OverPressureDevice (OPD) on newer small tanks (those having triangular valve handles).
On the 20-lb tank open the thumbscrew (or screwdriver valve), open the bulk tank valve first making sure all connections are tight, open small tank valve, fill until liquid squirts out the 80% thumbscrew. Close thumbscrew and close valves. Good to have second small tank standing by to hook hose to, open little tank valve with bulk tank valve closed, bleed liquid hose into mostly empty second small tank.
When cold temps may take 10-min to fill small tank, at 110-F happens quickly. Bulk tank pressure increases as air temp rises and creates more vapor pressure to more quickly fill small tank.
If you also fill tractor propane or truck tank or tanks other than 20-lb will need other adapters. Get the kind of adapter that has a handwheel doesn't require wrenches.
Your dealer should assist you as he sells more propane that way. Some get horsey about it saying only they can fill bottles and refusing to sell hoses and valves and adapters. If you own the tank (not leased and therefore tied to the one dealer) I'd find another dealer. If you own the tank you can also negotiate prices. If you lease the tank I'd buy it or find a good used one and move it yourself. Strap beneath and don't use the lifting ears if there is any liquid in the used tank. Dealer won't move a tank with liquid aboard.
You can find everything you need on the Internet but would be helpful to talk to somebody at sales desk that knows what they're doing. Try Arkansas Gas Equipment Co in Little Rock or look up Rego for valves on the Internet. I've had good luck with both, buying directly anything I needed.
Other poster was correct: not too many years ago every farmer in our part of the world had a liquid fill line off his home propane tank to fill his tractor and pickup. Starting to come back around but not like before. Used fill hose assemblies are getting hard to come by. In my area used large tanks sell for about a buck a gallon, sometimes much less, and you move it.
The stuff you need to acquire won't be cheap, but will last about as long as you have interest. I keep my 20-ft liquid fill line coiled around the tank and piece of old carpet thrown over it to keep the sun off it. White carpet to help keep tank cool in summer and when grass fire approaches easier to wet tank (tank shouldn't blow, but may vent worst case in small fire; look up BLEVE on web).
If you think your way into the process and pay attention it's no different than using your air compressor to fill a portable air tank. Just be careful.
Please post back your continuing experiences so we may all learn.
tractor...
