Once you have your fingers in place, you need to blow into the mouthpiece of the clarinet while simultaneously pressing down on the Bb and A keys with your left-hand index and middle fingers. Your right-hand index and middle fingers should also press down on the E and F keys, respectively, while your thumb lifts off of the register key. This combination of fingerings and blowing will produce the sound of an eighth note.
It is important to practice this fingering and blowing technique regularly in order to master the production of an eighth note on the clarinet.