Correct way to punish a puppy?

I turned around and saw my 6 1/2 week old puppy pooping behind the couch. I caught him in the act and smacked his bottom a few times. Not very hard at all but the poor fella yelped and skittered away. He's now laying at my feet, and I can tell he feels guilty. Is this the correct way to punish a puppy? I don't want him to grow up skittish.