The sound is hard to explain but is easy to do (make a 'k' sound immediately followed with a long (1 second) 's' sound). At the same time you have to sort of quickly lunge your face towards theirs so it scares them more adding to the sound.
Also to make it easier for you, pretend to be talking to someone that is relatively behind the person you are going to staunch so that you will be a tad closer to the staunchee so that they will have less reaction time (they will sometimes have time to see you begin the staunch and start to duck straight away).
If you obviously see them flinch or blink massively then you yell "STAAAUUNCHEEDD!!!" and laugh loud.
Staunchee: *mad flinches or something*