You've "assumed" about what I've heard about it. There is a nice (and fairly clear) picture of what one might presume to be a "conventional" player at Carmen de Vicente's homepage, showing the strings around the thumbs. She gives lessons...

My impression is that castanets are conventionally played only by the dancer(s) - and the technique appears to be part of several "dance class" offerings, but doesn't appear much as a separate "subject."

You can get "castanets on a stick" (like the riveted-together spoons ) and "castanet machines" so the percussion section of your orchestra can get the sound without actually learning to play traditional castanets.

The obvious first answer to the question "how do you play castanets?" would be "why would you want to?," but that's the result of their frequent appearance in the hands of vagrant urchins who's (non-musical) parents bought them to "shut the snot-nosed-little-brats up" at the "junk booth" at the festival. They're usually one of the cheaper items for sale.

The serious side of "why would you want to" is - "what are the legitimate and traditional uses for the instrument?." They seem to be commonly used now almost exclusively by "Spanish dancers," although they appear in the work of a number of "Orientalist" painters as though they are a "Harem dancer" accessory - implying a rather different (more "eastern") origin.

I recall a third(?) grade teacher using hers for "mob control." A quick click would instantly get the attention of the class, and a "roll" meant somebody was in deep s... I've also seen them used by "elevator attendants" to signal arrivals and to "dispatch" the next car - back in the days of manually operated and attended elevators. In at least one restaurant I visited in the early 50s the "mayter dee" used castanets to signal the waiters (fancy restaurant, no "waitresses").

But I'd be interested in anyone's comment on musical traditions of the things.

John