When you arrive in Nantes take the North ring road.
After the Cheviré bridge, follow Noirmoutier then St Brévin les Pins
On leaving Vue, turn left towards St Brévin before St Père en Retz, turn left at the corner of the Terrena store, follow this road of menhirs. In the hamlet La Tendonnerie in a bend, take the unpaved road with a red post box.