This exercise is best carried out with a metronome set to 60 beats per minute (1 beat per second).
To develop your breath control spend a few minutes every day holding long notes. Your aim should be to sustain notes without undue wavering, with a firm tone and with secure intonation. The tone should be firm but not strong. The pitch of the note must be steady.
First, fill your lungs with air. Inhale through your mouth and not through your nose. Good breathing should be silent. Your lungs are shaped like two pears, the greatest volume lying at the bottom of the lung. You must use your diaphragm to fill the lower region of your lungs. Place one hand firmly against the area just below your stomach and breath in deeply. You will feel outward pressure against your hand as the diaphragm moves downward to create a negative pressure around your lungs which will 'draw in' air through your mouth. Keep your throat relaxed when you breath. When the lungs are full, start the note. Hold it steady, with a good firm tone, neither very loud nor very soft. Stop the note cleanly when you feel you no longer have enough air to sustain it.
You can play along with the music sample above. With its default setting the note should last about 2 seconds. If you do not want to use the music sample above, chose a note and play against a clock. You will find that high notes and those in the middle of the instrument consume air much faster than low notes. You must be aware of these differences so that when you play slow sustained melodies you do not find yourself running out of breath in the middle of phrases.