How do you teach a dog to Growl and Snarl on command?

I want to train my dog this so I can freak out anybody who looks like a threat. (Although, if they are a very serious threat, my dog won't help anything) I want to train him but I have no clue how! I don't wnat to (like some bozo said) beat him till he growls and train him that way. Sheesh--he'd kill ME! lolz. Anyhow, any idea on how to do it? Thanks!!