Since the snapshot has already been taken, you will just have to wait for ca way to officially claim your bitcoin diamond by sweeping your private key into a supported wallet. As far as i know there are no ways of doing it right now.

I am aware of a few exchanges that are distributing bitcoin diamond, however since the snapshot has already passed it's too late to put your BTC in there.

If it's just a small amount, you probably shouldn't even bother. The transaction fees of clearing your BTC address for security is going to be not worth it for the reward.