To send a signed message, compose a new email and click the Security icon. This icon looks like a large padlock. Choose "Digitally Sign This Message" and your email will be signed with your digital signature.
To send an encrypted message, compose a new email and click the Security icon. This icon looks like a large padlock. Choose "Encrypt This Message" and your email will be encrypted. You can only send encrypted emails if your recipient has already sent you a signed message.