de Paris:

A6 direction Lyon
Exit Auxerre
Direction Auxerre
Direction Nevers via the N151 20 kilometres after Auxerre
Brocante, turn right after 2 kilometres

de Lyon:

A6 direction Paris, exit Avallon, in Avallon,
follow the signs for Auxerre, about 20km after Avallon in Voutenay sur Cure, turn left in the direction of Courson les Carrieres, go to Courson les Carrieres
(about 25km), in Courson les Carrieres, turn right in the direction of Auxerre, after 2km, turn left in the direction of Fontenailles

there you are...

GPS Coordinates :
N 047° 37.50'
E 003° 27.52'