I may be wrong, but I've always been led to believe that a wing fillet is actually the small piece of balsa or whatever that's glued to the trailing edges of the wings and the fuselage to fill(et!) in the gap there, the purpose of which is to help make the trailing edges curve and 'blend in' to the fuselage. The curved piece that goes over the upper surface of the wing and against the fuselage side is a fairing.
Read the article on wing fillets, and just installed them, but made some slight changes: I used a little heavier card stock than tracing paper, and laminated silkspan on one side. I left about a quarter of an inch of silkspan outside the fillet border. This allows you to install fillet by either doping or 50/50 glue mixture with the silkspan now being attached directly to the fuselage, and wing. It blends in beautifully with no step showing. Very little sanding necessary.