The IPA symbol is /ɬ/. If you're struggling to say it, try saying "kettle". Keep your tongue around the position of the "ttle" and say "ttl" a few times, then try blowing out at the end of the sound. After a while the blowing sound should get smoother and you should be able to say it without the preceding "ttl". Then it's just a matter of slightly adjusting your tongue until it sounds juuuuust right. (I learned to say LL using "kettle" when I was in primary school, and I'm not good at explaining but... the method worked!)
The way I started to learn it was by saying 'l', but blowing air over my tongue like I was saying 'sh' at the same time. But skuggstralar's suggestion sounds good too.
The main thing is remembering that it isn't said with any particular force or "whispered" like some guides say. It's actually said in the same way as any other sound.
See what works for you in here as well: http://gwybodiadur.tripod.com/ll.htm
Similar but not quite. Skuggstralar does a good explanation of the Welsh sound above. You can compare the recordings Wikipedia has for the sounds.
Icelandic 'll': /t͡ɬ/ https://en.wikipedia.org/wiki/Voiceless_alveolar_lateral_affricate
Although single 'l' is /ɬ/ at the ends of syllables in Icelandic: siglt /sɪɬt/, which could help if you know Icelandic.
This came up saying I had a typo as I said "I'm eating meat and vegetables" and instead thought I meant to type "I'm eating meat and veggies".
Since veggies isn't really a word in English and slang for vegetables it shouldn't really be the "correct" answer should it?