Agood way to start is to put the two loops over the thumb as Liz describes above then imagine you're sitting at a lunch counter waiting impatiently for the waitress. Then start drumming your fingers on the counter. Start with the little finger of the left hand and tap the shell of the castinet just hard enough to make the two halves come together to make a clack sound. Then tap with the next (ring) finger, then the middle finger then the index finger. Then repeat this with the right hand. Then the left hand again. When you can finally do it fast enough and evensounding enough, you've done a tremolo or roll like a drum. To accompany music thus tap youur fingers on the castanets to the music. Or if you read music, there's music available to show youu when to tap the castanet. That's it in a nut shell as basics. It's more involved than this but you can get started and develop technique this way, at least.
walt