Wait till they start to dry out. They'll pop open.

Pura Vida

Here is a link that might be useful: garden blog

See the last photo on the page for what the seeds will look like when they're ready.

Here is a link that might be useful: castor bean bloom and seeds

Hope you know they're poisonous, so keep kids and pets away.

Thanks for the tips, everyone. I'm excited to say I've extracted lots of seeds already and more are maturing every day! It's much easier than I anticipated. Looking forward to growing them next year! :)