A back that flares outward is upholstered from the top down, which is the reverse of normal practice.
On a standard back, the upholsterer works from the bottom, tensioning upward, because gravity and the shape of the frame both help. On a flaring back, working upward means dragging the covering into an ever-widening space - the cloth runs out of width before it reaches the shoulders, and the last stretch is under tension in exactly the place it should be relaxed.
So the covering here is fixed at the top edge first, at the widest point, and worked downward into the narrowing frame. The fullness that gathers as the frame narrows is taken up along the sides, where the gilded frame will cover it, rather than being fought across the face of the panel.
The damask is also positioned before any of that: a full motif centred at shoulder height, not at the middle of the panel, because on a flared back the shoulder line is what the eye reads first.