One comfortable pitch on a vowel for the length of one exhale, ten times, then narrow the vowel and hunt the harmonic. A live spectrum while you sing, a count of exhales, and a date. The spectrum shows the harmonic and says nothing about what it means.
The exhales
The spectrum is live and is not recorded. Without a microphone the practice is unchanged: you can hear the harmonic yourself, which is the point.
Exhales today
0
Ten is the usual count and nothing enforces it.
Days
The honest limit, stated first. Nobody alive has heard medieval chant. The authentic ancient sound on the albums that sold millions is a nineteenth century reconstruction from manuscripts that do not record rhythm, made at one abbey, adopted by the Vatican in 1903, and substantially revised from inside that same abbey in 1968 by a monk whose semiology contradicted his own house's rhythmic method. The story about a monastery that fell ill when it stopped chanting and recovered when it resumed names no abbey, no date, no measurement and no record, it was carried into English in a 1991 anthology and a 1997 bestseller, the organization built on that researcher's name no longer tells it, and a Council document of 1963 says the opposite of its premise. Overtone singing is a matter of shifting the resonances of your own vocal tract to amplify a harmonic that is already there. It is teachable in weeks and evidences no altered state. You cannot recover the sound. You can only keep singing.