Click dropdown menu to the right of the calendar you want to share.
Select "Share this Calendar"
In the box below "Share with specific people" add the email address firstname.lastname@example.org. Under "Permission Settings" choose "Make changes to events" and click the button: add person. Then hit one of the save buttons.
To ensure that you have added your calendar correctly, go back into "Share this Calendar" (instructions above) and verify that appointment.set.one@gmail is listed as seen below. If it does, you are done!