How to speak with a London accent?

I'm English, I'm from the north so I don't speak Queen's English, but I would suggest buying an audio book - the Harry Potter one is read by Stephen Fry who has a very proper accent. When you say London I'm uming you don't mean ney, youtube 'Eastenders' and you'll see why you don't want to talk like that. As you're American, you probably speak very nasally, in England we speak more from the back of the throat. Good luck with it!