The clarinet should be pointing toward the ground at a 35- to 45-degree angle. The exact angle will change slightly among players. Players with an extended lower jaw will need a greater angle, while those with an inward sloping jaw will need less of an angle. The reed of the mouthpiece should be facing the floor, and the actual mouthpiece should extend into the mouth about 1/4 of an inch.
The mouth needs to form a tight seal with the tip of the mouthpiece. To do this, you must tense the muscles in your cheeks. Imagine you are sucking through a straw to form a proper embouchure. Elevate the tongue inside the mouth slightly to create a ramp for the airstream to travel. Place the tip of the tongue close enough to the reed to use the tip for articulations.
The upper teeth will make contact with the top of the mouthpiece. The upper lip then forms a seal between the teeth and mouthpiece. This helps ensure that air does not leak through the sides of the lips. To prevent breaking the reed, the bottom lip curls over the bottom teeth and forms a tight bind with the reed and mouthpiece tip. Flatten the muscles at the front of the chin and point your chin slightly downwards.
Pronounce the vowel sound "ah" to open your throat. When the throat constricts the airflow will be hampered. To prevent this, you must keep your neck relaxed and sit up straight. Poor posture will create tension in your neck and shoulders, preventing you from forming a proper embouchure and appropriate tone production. Try finding the position that gives you the freest sound while pronouncing "ah." Do this by slowly rotating your neck up and down until you find the position that works best.