In Mail, click "New email" and then enter all of the details for your email. Click the lock icon at the bottom of the window. The lock will close and it will ask you to enter your Guard security password. Then click "OK". Now just click "Send" and your email will be sent encrypted.
Tip: Click the "Options" icon for additional features for sending your encrypted email. You can choose to sign your email or attach your public key from the dropdown menu.