Close your mouth so that your lips are just barely touching and your teeth rest lightly against each other. Now take a deep breath, relax, and starting mid-way through your range, vocalize an "ooo" sound followed by an "eee" sound, all the while avoiding the temptation to puff out your cheeks or let too much air escape.
When you run out of breath, try to breathe in without losing the mouth-shape you've found, then repeat. Your teeth should be buzzing a little - let the tone go lower, lower still, until you find a note that makes your entire head feel like it's going to start to vibrate.
This is your own resonant frequency, your personal drone-tone.
My note is C# two octaves below middle C. My resonant frequency, my natural drone-note, makes my teeth buzz against each other like they've been electrified, especially on the "ooo" vocalization. It also helps clear my head, and makes strangers look at me like I'm the stranger. Pshaw.
(You'll probably get a buzz-effect at a bunch of places along the way, as you will at octave intervals above and below your personal-best drone. But one of them will be the best - find that one.)
It's hard to create good solid tones - and hold them. Unison singing (plainsong or Gregorian chant come to mind) is damn hard, harder than throwing down another few bars of Handel's Messiah. Singing over a drone can be especially challenging. Ever sing accompanied by a bunch of bagpipers? Not easy.
(Remember the definition of a gentleman is someone who knows how to play the bagpipes but doesn't.)
Drone music - from Fripp & Eno to Autechre to Japanese gagaku to Australian didgeridoo - makes you slow down and listen. It forces you to contemplate the unchanging now. And it's easy to do: anyone can pump the bellows on a harmonium or turn the wheel on a hurdy gurdy and drone away. Just like anyone can do what I described at the top of this post. It's a path to 和 (wa), a vehicle for harmony, peace, balance. One note, whatever that note happens to be.