Best to find some videos and such online. You have to put your tongue in an L position, but then push air around both sides of your tongue.
For listening exercises, you have to type what you hear.
If the voice says llaeth, type llaeth.
If the voice says llefrith, type llefrith.
In both cases, it’s irrelevant what you yourself would say.
Llaeth in South Wales. Llefryth in North Wales. So many words are different. I wish there was an option to learn North Wales Welsh!
The spelling is llefrith in parts of north and mid-Wales.
There is no particular 'north Wales Welsh' - there are two or three main dialects in 'north' Wales, depending on where you think the 'north' is...
The notes for the section 'Dialects' have some links to more information about the various Welsh dialects.