Like lcjones says, take a simple little tune and substitute the words with something really evil....
Say, for example... Twinkle Little Star...
Something like this...
Itchy itchy little scar
never should have broke that jar
Never should have gone barefoot
dancing naked on that bar
Little scar why do you itch
man I tell you it's a ..... 
Well, you get the drift...!!!
No matter what you attempt in life, always try to have a little fun at it....!!!
Cheers...!!!
Ben