How to do beatbox technique, polyhonic overtone singing?

Sing and elongate the letter 'e' in your singing voice
Transition from the 'ee' sound to an 'ooh' sound, changing your mouth position
Pull your tongue back to the back of your throat to create more space and will amplify the overtone
Reposition your tongue while singing the 'ee' to 'ooh' to find the sweet spot and you hear the overtone sound
Continue changing your mouth positions and the position of your tongue to hit different overtone notes