Singing is placing music on the breath.
Breathing is a matter of never-ending visualization, exercise and practice. We practice by alternating between taking in air slowly and quickly, holding it as long as possible, and regulating its rapid or gradual release. Breathing should not draw attention to itself but be relaxed and effortless. Deep breathing is by nature relaxing to the body. Shallow and quick breathing is invigorating and energizing.
A teacher of mine once suggested that the ultimate goal is to sing through 24 measures on a single breath. Learning how best to breathe is the challenge that every song provides.
Knowing where to breathe is easy when one is singing for words, for then the natural breaks reveal themselves automatically in the cadence of the language. To help me identify the best places to breathe inside a song, I speak the text aloud as I am learning it. As much as skiing is just another way of walking, singing is just another way of talking.
All the air we need to sing with is there. It exists in abundance in the space around our heads, our bodies. And all the time we need for getting the right amount of air is always there as well. Trusting in this comes simply with experience.