It's a voiceless lateral fricative: the
tongue-tip rests against the upper gum, and the air is forced down the side (or
both sides) of the mouth between the tongue and the upper side teeth. The effect
is not unlike "th": British English speakers often substitute "thl"
for Welsh <ll> in placenames ("Thlandudno").
My tongue has always tended to stroke the upper side teeth
when making these fricatives. The tongue is spread wide at the front of
the mouth, resting just under the teeth, almost as if I was going to
'scratch' my tongue with my upper teeth. I forget what Welsh orthographic
ll stands for; for some reason, my memory insists it should be like the
double L in llama, but this cannot be