...I interpret your meaning to be "with as neutral an expression" as possible, i.e. with as relaxed facial muscles as possible with minimal brain thought driving the facial muscles, as we trickle the air out through the nose.
I think I can get the "face as silly as possible" part -- maybe I'm doing it already, but I sometimes have a little trouble with the slow air trickle...
Yes, you understood exactly what I tried to say :-)
Think it's not necessary to exhale exclusively through the nose. If you don't care of your facial muscles and let them relax, your mouth may be a little open. Just be aware water doesn't enter nose and mouth and don't mind some bubbles. Doing so should result in your strived "air trickle"...
Enjoy game and sound of the bubbles and you must not show a face like for an application photo...