If you have started a link from a People card, clicking in the proper field
(Husband or Wife for "To Children"; a child field for "To Parents"), will allow you to link back to the People card.
Otherwise, clicking in an empty field, or option-clicking in a filled field allows you to start a new link. Note that you can not leave blank children: You must enter #1 before #2, etc. After you have found or created the person to whom you are linking, click on LinkBack to complete the link*.
Starting a new link at a relation card gives you the additional option of a 'no detail' person. This allows you to type a name, or'daughter' or 'Died at birth' or something similar for that person. 'No Detail' fields appear in parenthesis.