How to sing overtones

Overtone singing is essentially a highly refined articulation of vowels, and thereby tone colour. Tone colour is determined by the automatically resonating overtones that vary per vowel.
You may try for yourself by singing the word ‘bird’ at one constant pitch. If you hold the ‘r’ and very gradually shift towards the ‘i’ you can hear the tone colour brightening up. When you gradually move from ‘i’ to ‘r the reverse happens. You can also practice with the words ‘tree’, ‘free’ (ascending overtones) and ‘ear’, ‘near , ‘dear’ (descending overtones).