I have a few issues with teaching "Quiet" to stop barking.
1) not all barking is intentional - it can also be an emotional response that's not necessarily under the dog's control - like laughing or crying in humans. In order to stop the barking in this instance, you have to identify and change (or at least reduce) the underlying emotional response.
2) dogs are totally capable of learning that what they are getting rewarded for is stopping barking - which requires starting to bark in the first place.
But, if you do want to teach a quiet, the way to do it is to let the dog bark, and then say "yes!" and give him a cookie for stopping. Eventually the dog will start to expect a cookie when he stops barking and will start to look at you, and you can start to put a hand signal on it. Simple in principle, much more difficult in practice. Easier to teach the dog not to bark in the first place, IME.