How do professors learn how to write letters of recommendation? Writing letters of recommendation must be a fine art. Culture across fields and countries vary wildly in almost any respect, and this is likely to the case for LoR.

How/when do professors learn how to appropriately convey the value of the person they recommend?


As they write them, by trial and error seeing which students land the position. But this would require a large number of trials, given so many factors come into play (including circumstances that have nothing to do with letter writing)
As they read them, while sitting on admission committee. I am then almost completely oblivious to the procedures, but as above, many observation might be required before one can be confident on its ability to write a proper letter.
As with most professorial duties, it's a sink-or-swim process.  There is no formal or official training they receive for this task.  They're just thrown into the situation with no experience and are expected to perform well at it immediately.  If they've seen other recommendations during their other duties (that they likely also weren't trained for) they can use that, otherwise they can try asking colleagues or do google searches.  So for the most part it's a bunch of untrained people relying on other untrained people and crossing their fingers that the other guys were able to figure out a good approach.