So often, we think of the arms and shoulders when trying to correct topline. Setting/Rolling the shoulders back all happens in the back as LG and a couple of others have mentioned. Pulling the lats in and downwards is easy, actually. Relaxing the arms is th hard part. With the arms in a relaxed, lowered, and slightly rounded shape (first position), reach lightly down and outward as if scooping water from a basin. Lift the arms continuing to reach down/outwards. When the arms get to about chest level, slightly turn the palms inward as if to let the water pour out of the hands.Notice how this naturally begins to open the embrace. Slightly part the arms to about the point where the hands are in front of the pecs. It is here that the shoulders want to start to rise up. Maintain the in/downward pull in the back, and only adjust the hold from the elbows. Hope it conveys as well here as it works in class. Bonne chance.