I have been assured by a Welsh-speaking friend that hl is pronounced
around the same as the ll in Welsh - this is effectively done by this manner

lips slightly parted and the tip of the tongue behind the two front teeth of
the upper jaw
now try to hiss [...]

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Try for a gutteral "hiss" - kind of at the same "vibration" as the Scottish
"ch" ("loch"), but at the top-front third of the palate - and try to say
"h(almost-pronounced ch)ll". Pretend you're an angry cat trying to say "L".
;-)

That said, I'm not sure "ll" is quite accurate for "hl"; similar perhaps,
but more "plegmy" sounding and less "hissy".

Em
(spoke Welsh before English, mostly forgotten now through lack of use)