By car: set your GPS to "mairie de Monnetier-Mornex" (Monnetier-Mornex town hall), the "chemin du vallon" (valley path) is 300 metres after passing in front of the town hall (the path starts beside the little chapel on the large bend).
By public transport: from Annemasse, take line H (Proximiti) to the Mornex Chapelle stop, then walk 500m.