The route is somewhat visible from the Chanrion Hut and, going down gently undulating pastures you will reach the Grand Charmotane bridge, then cross it. After a decidedly steep stretch, the slope gets gentler and goes via rocky, grassy areas, then a nice path will take you to the Fenêtre de Durand. This col is where Luigi Einaudi was welcomed by Swiss soldiers back in 1943, fleeing from Fascism; after having profited from Swiss hospitality, this eminent political figure would then become the second president of the Italian Republic after the end of WW2.
From the Fenêtre de Durand, the first part of the route towards By is dominated by the austere size of Mont Gelé and the gentler shapes of Mont Avril, which make up the gateway between the Valais and the Aosta Valley. The waymarked path first crosses a stony ground and then reaches the Thoules pastures. That is where the forestry road leading to the Balme pasture and then the By Basin departs; from there, the path climbs up 150m and then reaches the Ru de By, that you should follow as far as Néan. Leave the Ru and climb to the Tsa de Champillon, then reach the Letey Hut.