From Montanges, take the D14 towards Chézéry-Forens. Park at the stream of the same name near the bus stop.
(S/E) Before the bridge, take a tarmac path on the left which quickly becomes uphill (a sign indicates access to Crêt de Chalam in 3 hours). Along the way, you will notice the entrance to a cave, which is blocked. An unpaved path overlooking the stream continues uphill. You will come to an intersection.
(1) A sign indicates the Cascade des Étrés waterfall 10 minutes to the right.
Turn right to reach it.
(2) Turn around.
(1) Continue to the right, straight ahead towards Crêt de Chalam. Cross the bridge (there is a stop sign followed by a no entry sign). The path begins to slope more steeply. Continue to Petit Mannet.
(3) A path to the left of the house begins to climb steeply (with the advantage of being in the woods). A little further on, you will pass the ruins of Grand Mannet.
(4) Continue climbing for a while on the steep path, still in the woods. Once you leave the forest, remember that the summit is not too far away. You will reach it via a few steps in fair condition. At the crossroads, keep to the right until you reach the summit.
(5) Enjoy the view and the two orientation tables: to the east, you can see the imposing Jura mountain range in the foreground (Crêt de la Neige - 1,720 m, Le Reculet - 1,718 m, Colomby de Gex - 1,688 m, La Dôle - 1,677 m, etc.). If you're lucky, you may even be able to see Mont Blanc."
The return journey follows the same route back to the car park (S/E).