Buy quality fittings to match the two tanks' valves and a good hose in between, maybe a separate 90° open/close valve if you like.
At a propane dealer they can help, authentic Rego brand are best, lots of cheap chinese crap out there. Otherwise Tractor Supply, maybe Home Depot, link below are just for pics.
Helps to know the terminology
QCC Type 1 or QCC1, ACME POL are keywords for the standard outlet on a BBQ 20# tank, the old school POL part is female, the ACME 1-1/4" is male can use either style.
Most other-size bottles these days use the same, but YMMV
example with overkill
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"Coleman style" if you use the 1# refillable DOT-approved version of those, valve referred to as 1"-20
example adapter made up
Amazon.com : MENSI New Updated Propane Refill Adapter Hose for 1 LB Gas Bottle 35.5" Long with ON/Off Control Valve (QCC Type1 Inlet for New Tank) : Garden & Outdoor
www.amazon.com
So just suspend the source tank upside down above the target OUTSIDE no sparks nearby of course check for leaks soap test.
Example stand, but can easily rig your own DIY
The Flame King 1 LB (16.4oz) refillable propane cylinder and Refill Kit with its CGA600 connection, fits all appliances that today's non-refillable tanks work with. This propane bottle is legally & lawfully
www.homedepot.com
Also refillable 1# there.
Not all the gas will transfer, warming the source a bit will help get close to empty if going to exchange it.
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Note can go anything to anything these days, also go between butane and propane nice adapters from Korea.
Those portable Iwatani style burners are great, can run off plumbed propane when installed, then toss them in a backpack for a picnic using the $1 canisters from Asian shops
