Since it is slang, you really never have to use it, but if you want to, use it anyplace where you would say "isn't it" or "aren't they". You could also use it (in Britain) to express agreement, in the same way as we might say "isn't that so!"

It isn't even exactly slang, but just the lazy sound of lazy speech, slurred speech - just like 'gotta' and 'gonna' and 'wanna' so there are some new ones but if ur gonna use em it's pretty ugly, innit? try some stine, while you're at it, owboutit?