how exactly does one tune a theremin?

well...?

Well, you are in luck with a Theremin. Every model, even the least expensive, is a multi-temperament instrument. :)

OK... seriously...

All Theremins have a front-panel tuning knob that one uses to set the tuning. You adjust it so that the pitch rises as you move your hand towards the pitch antenna. As you move away from the pitch antenna, the pitch will decrease until the Theremin becomes silent. Thereminists call this "zero beat" -- meaning that the difference between the frequencies of the reference and sense oscillators within the Theremin is zero.

The "zero beat" point occurs at a certain distance from the pitch antenna -- determined by the setting of the pitch knob. Many Thereminists set this distance to coinicide with the place where they position themselves for playing so that moving the pitch hand from ones shoulder to the antenna engages the full range of pitches.

On some Theremins, you may have to tune zero beat to be behind you... An antenna that has inferior linearity may have a limited "sweet spot" (the range wherein the note spacing and linearity allow precision playing) in which to play so you may have to adjust the tuning to bring the pitch range that you want into the "sweet spot". As you move into more expensive (more linear) Theremins, the sweet spot covers a greater distance, and thus, a larger range of notes is playable with a given pitch knob setting.