Try this link: http://gwybodiadur.co.uk/. The most important thing to remember is that it's not as difficult to do as a lot of resources seem to suggest. Any unfamiliar sound can be difficult if the resource makes it seem difficult.
I like to think of the ll sound as putting your tongue in the position you use for "l", but saying "sh" instead (keep you tongue in the "l" position, rather than pointing it down like you normally would for "sh"). It doesn't need to be said with any great force. But there are other ways of imagining it, like in the link.