Sung Breathing
Sung breathing is a very simple process but can deliver quite profound results. At this point we won't cover the whole process, instead we will focus on the first two stages.
Feeling the breath:
Before we can work with the breath we need to feel it. The basic idea is that we bring our awareness inside and begin to feel the breath. This includes feeling the nostrils, the throat, the air as

it moves down into the lungs and as the abdomen expands in all directions. We fully engage our awareness with the breath, without judgement, without trying to force anything, let it be natural. Try to feel as if you are observing the breath, the mind is gently following the movements. As we do this the breath will naturally deepen and become more relaxed. Now we are ready tostart.
Breath in tranquility:
Breathing in tranquility refers to the sensations felt in the body during inhalation. That part of the breath that gives us life, that restores and energizes us. As we breath in feel the body and how it responds to the breath. You may feel warmth or tingling, whatever you feel allow it. Fill the body with the breath, let the feeling of tranquility nourish each cell. Always remember to be gentle with this process, don't push or become to serious, practice in my opinion should be light hearted, almost playful.

Breath out tension:
Now you are fully engaged with your breath you'll probably start feeling tension in certain parts of the body. Now we begin to release the tension with the out breath. Remember this is not a visualisation, although a little visualization can help. This is primarily a physical exercise.
We really want to feel the physical body relax and let go. Of course by this stage of the breathing you should already be very relaxed so you should naturally take it gently. Forcing or concentrating to intently causes tension in the body and so is counter productive, that's why a childlike approach is best.
Where you begin to release tension from first is up to you, some teachers advise starting at the top of the head, others the chest, maybe allow your own body to tell where first.
And beyond...
The next stage after this is called breathing Heaven and Earth, this involves feeling more deeply into the body, but also the Qi or energy of the body. Its advised to follow the first stages for at least a few months before moving on. My own feeling is this is a natural process, once you've relaxed enough and moved deeply enough within you will naturally begin to feel and be ready for the next stage.