You will notice when you begin to sing, if you watch yourself very carefully, that, first, you will try to inhale too much air; secondly, you will either force it all out at once, making a breathy note, or in trying to control the flow of air by the diaphragm you will suddenly cease to send it forth at all and will be making the sound by pressure from the throat.
There must never be any pressure from the throat. The sound must be made from the continued flow of air.
You must learn to control this flow of air, so that no muscular action of the throat can shut it off.
Open the throat wide and start your note by the pressure breath. The physical sensation should be first an effort on the part of the diaphragm to press the air up against the chest box, then the sensation of a perfectly open throat, and, lastly, the sensation that the air is passing freely into the cavities of the head.
The quantity of sound is controlled by the breath.
In diminishing the tone the opening of the throat remains the same. Only the quantity of breath given forth is diminished. That is done by the diaphragm muscles.
"Filare la voce," to spin the voice from a tiny little thread into a breadth of sound and then diminish again, is one of the most beautiful effects in singing.
It is accomplished by the control of the breath, and its perfect accomplishment means the complete mastery of the greatest difficulty in learning to sing.
I think one of the best exercises for learning to control the voice by first getting control of the breath is to stand erect in a well-ventilated room or out of doors and slowly snuff in air through the nostrils, inhaling in little puffs, as if you were smelling something.
Take just a little bit of air at a time and feel as if you were filling the very bottom of your lungs and also the back of your lungs.