Stand with the clarinet at an angle of 45 degrees to the body. Sitting is not recommended when learning, as it constricts airflow.
Stretch your lower lip over your bottom teeth. This creates a cushion for the reed and will prevent air from leaking through the sides.
Place the clarinet in your mouth with the reed touching the bottom lip. Use the end joint of your thumb as a reference for how far the clarinet should be placed inside the mouth. Your teeth will rest on top of the mouthpiece and the upper lip will create a seal with the mouthpiece.
Breathe from the diaphragm -- taking air in through the corners of your mouth -- and pushing it down, until your stomach and sides grow large with air. In fact, the act of simply expanding your stomach will force you to take air in.
Push the air out of your body by contracting the muscles of the diaphragm. Blow a slow and steady stream of air through your mouth. Keep the tongue slightly arched to create a ramp for the air to travel across -- and through the reed.