Sing any low note, near the bottom of your vocal range. The partial overtones are all in there, and make up the timbre of your voice. By shaping your mouth, throat, and pharynx cavity in the same way as you do when you whistle a tune, you can emphasize any of the partials of the low note that is the fundamental tone. In fact, the vocal and other resonant cavities will be the same as if you were whistling the tone you're attempting to produce.
The easiest ones to produce are in the second and third harmonic series. In the second harmonic series you'll find the third, fifth and octave of the fundamental. In the third harmonic octave, you'll have those along with additional tones. Above that, you can actually produce a decent scale, but that's a bit more difficult and takes a lot of practice.
It took me a few minutes the first time I tried it, after hearing the Tuvan throat singers, but once I produced the first overtone, the rest came easily. Very small changes in the resonant cavity you produce are required.
Try it. You can do it, I guarantee, if you're able to carry a tune with your voice. Just keep in mind that people will think you're odd, so try it by yourself until you get the technique.
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