By car, you will arrive either by the Col de la Chavade (via Le Bez, Loubaresse, then Valgorge), or by Joyeuse or Largentière (taking the direction of Valgorge).
In both cases, you will arrive on the D24. See map above.
Leave the D24 to take the eastern branch of the D124, which will take you directly to Monteil (2 km). If you take the western branch of the D124, you will go completely around the hamlets of Laboule (Le Serre, Les Abriges, Laboule town center, then Le Monteil: it's longer, 7 km!)