You don't need to do anything in terms of the actual ceremony, and no-one will know you areher godfather, apart from your family. However, even though you arn't necessarily a "practising" catholic it will be nice for you to make a point of it being special, by getting her a card with "to my god daughter" and maybe buying her some nice rosary beads or a cross and chain, which are a little bit more special than some of the tackier soveniers. She will keep them forever and it marks the point, that you have remembered your "vows" as a godfather. There are some lovely rosary beads around that girls and women admire, even if they arn't religious. Have a great day, and make sure you sing.