David Seidel covers how to do it pretty well. I will just add a little
to what he says. I never warm the big tank, but would if it was below
say 60 degrees F. I don't vent the little tank by pulling on the
pressure relief valve. If you do that, you may get some ice or
something prevents it from closing completely. Not a big hazard, but a
slow leak. I have always gotten it to seal again after some playing
around with it, but decided it was easier not to mess with it.

I put the little tank in the freezer before filling it. If it is
completely empty, it warms up rapidly when the new propane goes in. So
you don't get much propane into the little tank. So chill it again and
you will get a lot more the second time. Do it one more time and you
are likely to get it completely full and have some coming out of the
pressure relief valve when it warms up. Which reminds me, I need to
refill the little tank I use for starting the wood stove before it gets
completely empty.

Dan