This can be done either when prompted on first install, by pressing the “enable camera” button in the control interface.
Select a flat surface, and ensure that your device cannot fall off
Change the position of your hand to control pitch and volume
(The TrueDepth camera starts detecting depth at around 15cm / 6" above the device)
AirSynth displays circles to guide the position of your hands for pitch and volume
Press the toggles on the control interface to switch to slider control