From Buiza the camino ascends to the Alto de San Antón, passing the Forcadas de San Antón — jagged rock formations whose name translates to "pitchfork," an aptly descriptive word. It's about 2.5 km up to the pitchfork. The descent on the far side is more pleasant, passing through a pine forest.
Before the alto, you'll see a yellow arrow pointing along a non-existent path along the stream. Ignore it. A steel arrow indicating "Mieres" is the one you want to follow.
At the end of the pine forest, the camino splits and the marking isn't obvious if you've been lost in thought. On your left is a small concrete block building for pumping water. The official camino turns left here and off the road.
If you miss this turn and follow the road, you'll end up heading toward Rodiezmo. Along that wrong path you'll reach the Cruz de la Salve, where the camino turns sharply left. Miss that turn too and you'll end up in Rodiezmo, which means either following the busy national road back or retracing your steps.