Place your thumb and 'pointer' finger just behind the jaw and apply firm pressure. Give an stern "NO". Hold for a few more seconds then let go.
I only ever 'tapped' my dogs as a 'punishment'. When I knew for a fact that THEY knew they were doing the wrong thing.
For example, Lady snatched food out of my hand and bit me on th way. I gave a stern "Agghhh NO" and tapped. I then held it out again and she took it gently.