Hi, The correct way to pronounce it would be placing the tongue against the palate like we do to pronounce the L, but in this case, the tongue puts much more pressure against the teeth. You could find some people who pronounce the Ll as an Y, but in this case it is because there are native Spanish speakers who speak Catalan with Spanish accent, and can't pronounce the sound correctly, in the north of Catalonia( in Girona) you could find people pronouncing the Ll like a Y or a Spanish (i), in this case is an autochthonous phonological phenomena, in the same way that the southern Occitan dialects do, so, for example, you could hear people in Girona saying Paia and not Palla(Straw) or Fuia and not Fulla(Leaf)

As far as I know (not a native speaker, but I have spent varying amounts of time in front of language exchange partners trying to get this sound correct), it is actually something in between a regular L sound and the Spanish Y. These recordings are clear (I think), see if you can hear what I mean: https://forvo.com/word/ca/llet/#ca