I saw an illustrated guide of how to do it, but it was so long ago I probably won't find it again. It's basically a lot like how you pronounce the R sound, except that your tongue takes on a bit of an S wave shape. If you look at a referee's whistle, you can get a fairly good idea what shape it is that you are forming inside your mouth. The shape of the whistle is to emphasize a specific frequency in the hiss noise made by blowing into it. Polyphonic overtone singing is pretty much exactly the same thing, but using voice instead of hiss to generate the complex tones.
And just like learning to whistle, at first you'll sound like a raving lunatic and maybe even come close to passing out from hyperventilating. But once you begin finding the ideal tongue placements for a pitch, you'll very rapidly learn more, and it'll be exactly as automatic as after you learn to whistle. They're very very similar things to do, and the difficulty is identical.
If you are tongue tied (as I am) you probably won't ever be able to do it right. But you should be able to coax out at least a bit of overtone.
Now go out and try it, and enjoy knowing your neighbours will be forever terrified of you.
For the throat singing it's the exact same technique beat boxers use to create the deep bass. Just try to make a exagurated sigh. Do so until you accidently do throat singing for a brief second, then see if you can recreate that for a longer duration.
It's all about trial and error! It's really not as hard to learn as it might seem :)
For an actual musical instrument (OK, many will debate that point!), The Jew's harp uses the same principle as well. I recall hearing a legend of a Japanese player who'd lost all their teeth holding an ax to their chin and using that to get a resonation going. I'm not entirely sure how that was meant to work.
Is it possible to learn to wolf-whistle as an adult? All the people I know who can told me they learned as children. I didn't and I wouldn't know where to start.
Edit: I'm not sure if I'm using the term "wolf whistle" right (not a native English speaker). I mean the kind of whistle one makes by putting the fingers in the mouth and sort of pressing the tongue down, and that sounds really loud.
The story is great, and the mix with blues is really interesting. The dude has the most thunderous voice I've ever heard. Subwoofer gold.
So I definitely agree that the level one might consider a solo song, with melody and changing chord progression and/or oblique harmony, sung by a single person in solo, is rather rare.