Well, you don't need much. The election was 71-68 in the electoral college in OTL. Easiest way: Pierre Adet (French ambassador to the US) keeps his stupid mouth shut. His full throated support of Jefferson helped to create the perception that Jefferson was a just a French lackey, a notion the Federalists were already pushing anyway. If he takes a more hands-off approach, or even supports Jefferson in a way that isn't quite so overly enthusiastic, it could easily swing the election to Jefferson.