To add an availability, click on a day in the calendar.  

Enter the start and end time.

Enter the location

Enter a brief note

Optionally define a recurrence

The items will now show on the availability calendar.

Now, when a user is scheduling time for you they will see a highlighted area representing available time.

To delete an item, click to open. If its a recurrence, select either open this or open original. From the edit dialog, click the delete button to delete the item