Animated Gif question

[echostation]

Hi Guys, I'm new to the whole making gifs thing but I see some people post up some hilarious Heathe Ledger Joker dancing Gifs (like they superimpose Heath's head onto some movie clip/footage of actors dancing) and it's pretty hilarious!

I was hoping to do the same thing, like copy and paste a still photo of someone's head onto say like a clip of a group of people dancing or flying a plane so that it makes it look like that person is flying the plane or dancing like those Heath Ledger Joker gifs.

Can anyone please explain or help me understand how to do this? I've searched the net but it's pretty confusing to me at least.