Usually strong eye contact a penetrating look or verbal is enough. From 8-12 weeks grab the scruff on the back of his neck (much like his mother would) and shake firmly with a “No!” Follow with down stay to reinforce your dominance.
For older puppies needing extreme correction like growling and showing aggression toward people start with dog in a sitting position with your left hand holding his collar. As you make eye contact cuff the underside of his mouth with your opened right hand, rapping him sharply several times as you say, “No!” The discipline should be firm enough to elicit a sharp yelp. Remember, use this correction with an older puppy only in the rarest of circumstances, and only when the puppy is emotionally strong enough to handle it.
Another more common discipline technique is to roll the puppy onto its back and grab it firmly around the throat while saying “No!” Shake the scruff and continue to hold eye contact and verbal reprimand. Hold the dog there until it stops struggling for a full 30 seconds to a minute. Its tail should be tucked, and arms folded in. If it tries to lick your arm or hand in submission this is good and you can release him. This is a common alpha dog reprimand to lower dogs in the pack.