Breathing through the nose with the glottis slightly narrowed, so the breath rushes audibly like a soft ocean sound. In a flow it sets your tempo: one breath, one movement.
The quickest route to the sound: open your mouth and exhale as if fogging a mirror. That long \"haaa\" comes from a slightly narrowed throat – keep exactly that feeling and close your mouth. Now the breath rushes through the nose. Two mistakes happen almost every time: squeezing so hard the throat rasps, and only making the sound on the exhale. Ujjayi should be long, not loud, and sound the same in both directions; four to five seconds each way is a decent starting point. Its real value isn't relaxation but honesty: when the sound breaks, you've outrun your own control. If you get light-headed you're forcing it – drop back to normal breathing.