So, "L" is quite a common sound for children to have difficulty with.It's quite common for children to say "y" instead of "L" up until about 4 years old.
Some adults try to get the child to stick out their tongue when they say "L" yet we, as adults don't do that ourselves, so here's the best way to encourage a good position:
Encourage your child to put the tip of their tongue just behind their upper teeth. If they have difficulty working out where that is, you can rub this area with your finger or a toothbrush, so they can feel where it is.
Then, all they have to do is make some voice! Once they have the sound, they can start to practise it by putting it with different vowels:
They can have fun, singing "la la la la" to the tune of "Baa Baa Black Sheep" then "loo loo loo loo" and so on!
Once they can do that, then it's easy to start using it within words, especially those words that are motivating for children (e.g. "lorry" for the transport-mad child, "lolly" for the sugar-craver, and "Liam" if that's your name!)