Breathing technique, for me, was learned instinctively. Let’s face it, if you want to sing long phrases with good control, you need to figure out your body (which includes your breathing) and ways to control the input and output of your breath.
Too much focus on your breath can sometimes cause you to push and tense up. Remember, everything about singing is a balancing act.
I suggest starting in a relaxed state. When you inhale (through your nose), sense the intake going all the way to your toes. Sense the rhythmic flow of your intake as your belly and groin area engage in this rhythm. Try to stay relaxed and comfortably balanced (with tall posture) with the amount of air you can intake at this effort level. In other words, find your ultimate relaxed state where you can do this exercise without being short of breath or lightheaded.
Close your eyes and relax into this buoyancy. Again, sense the rhythmic and relaxed manner in which you can breath deeply.
Going through this ritual regularly will get you in tune with your breath rhythm and cycle. Once you know you are taking the breath down in a controlled fashion, you can start to elongate the inhale and exhale process (through your nose). As you do this, visualize your abdominal muscles and upper stomach area and back muscles expanding (stretching) and contracting in rhythm with your breath. Sense the slow motion control you can manage as you engage your body in a fashion that will ultimately help you sing better.
If you are continuing to be light-headed or short of breath, then back up and focus on the point where you can manage the exercise at your effort level. Again, this is about becoming aware and in control of your body and breath. For some, it will be totally foreign because you have been shallow breathing for your entire life.
When in doubt, do this exercise when you are tired and lying down in bed. Notice your belly and relaxed state your body is in before you fall asleep. Try to capture this slow motion buoyant and relaxed feeling deep within your body. Then try to match your stomach, abdominal and back muscles with the rhythmic flow of your breath.
Hope this has helped you sense how your entire body should be involved in good singing. I don’t talk about breathing too much because if your vocal cords (onset) are not in good shape, then your breathing efforts simply get in the way. However, once you are mixing and getting good cord closure, your breath control is your ticket to control with volume, better tone, flexibility, endurance, stamina…..get the picture?