'Sung' roughly means 'relax.' Using natural abdominal breathing is the most relaxing method. When you begin an inhale, imagine descending where the diaphragm connects to the spine - I had an analogy once to imagine the spine as a train track, the train rolls from roughly the solar plexus down towards the sacrum on inhale - so this descending feeling continues for the duration of the inhale - and then gently backs up on the exhale. It was taught in the context of someone asked about breathing while laying down. The breath should not go as high as the heart or as low as the genitals. The front of the abdomen stays relaxed and will naturally, gently expand on the inhale and withdraw on the exhale. And the one last secret of it is to time the perineum in with the breath, relaxing gently on inhale, lifting gently on exhale - it may take a little effort to work on the timing since the perineum has a very short travel distance as compared to the other structures.
Just working on the timing of breath will yield excellent results - even when doing things in a 'sung' manner, there is still a point of maximal energetic potential for each piece (hint, look at yin yang symbol) - so in learning 'timing' you learn to make all of those relative energetic maximas all happen at once.
One other good key to that is to learn to not use the nostrils to breathe - this goes hand in hand with timing - normally when we breathe the sinuses expand and buffer the air pressure - but this expansion stimulates the olfactory nerve in the sinuses, so eventually work towards only using the diaphragm - there is basically a simple dynamic of the 3 structures, diaphragm, psoas, perineum, which accounts for the most fundamental energetics. That may be a little tough to start off with depending on how quickly you get these things, so treat that as a secondary point. That was just something that came out of my longevity breathing work and troubleshooting my own body. The end result is dropping the flow of air beneath the threshold of turbulence, can no longer feel yourself breathe, energy soars - so think of those quiet pauses if you exhale slowly and hold it - except it is ongoing and only limited by the focus of your attention.
I didnt mention posture, but that also plays a factor, if you are slumped over then you dont have as much room for the guts to move freely.
Even in mildly complex systems, any outcome is the wrong thing to target, with the process being where the focus should be.