I can suggest as to how I do that,
Propane is heavier than air so knowing that you stand it so that the opening is facing down.
I like to blow compressed air to accelerate the 'purging', so to speak.
Done it more than a few times! (still here, LOL)
B4 welding/cutting I like to run a flame (plumber's torch?) by the open threaded end.
Occasionally, but rarely, I might get a little 'poof' if there is any residual 'fumes' but harmless,.
Time and lots of moving air is the trick.