Hi, there are various methods of doing O-rings with vapour, some people get along with using their jaw by short quick movements of bringing the jaw back and forward but keeping the mouth in an O shape, some people keep their jaw in place and lay their tongue against the floor of their mouth and make a short quick cough, the quicker the force, the faster you will get it going.
how ever the density of the O-ring makes all the difference, and to get them big, get them as dense as you possibly can, and follow them behind with your hands to help them expand and move, youtube is your friend in teaching you the technique correctly, as said before, practice in a mirror, so you can see whats going right or wrong.