The airway I use with my flutes is cut into the bottom of the block. It directs the air stream from the rear chamber to the sharp edge of the sound hole (front edge, or fipple edge) this splits the air stream, making the whistle sound.

As holes are covered, the length of the sound chamber increases, changing the tone.

It is tied on with a deerhide strip. It should be kept from sticking by giving it a twisting motion, and can be removed to dry the inner chambers after long periods of playing by letting it dry open overnight.

The block's leading edge should be even with the rear edge of the sound hole(forward hole) there is a mark behind the block to help with alignment. Moving the block slightly forward or backward, can change the sound. Experiment, you may like the sound better.