How do I attach fondant pieces to buttercream icing? Do I use water?

No, it'll stick to the buttercream itself, well as long as the buttercream is still moist, and hasn't dried up.

the best method is to pipe an open circle of b'cream to the back of the piece......if it's a big piece make 2 of them.
The open circle forms a suction action to help hold it better.

wow kakeladi! I learn something new every day! I'm going to try an open circle next time!!

Melted chocolate works very well too.