Why the still comes before the walk
A catwalk clip is an animated photograph. The video model takes the on-model still you give it and invents the next five seconds. It never goes back to your flat lay, so anything wrong in the still, a hem that’s too short or a missing button, carries straight into the video, where it is moving and harder to spot.
That makes the order of work simple. Get the still right first and check it against the garment as carefully as you would a product photo. Only then animate it. It saves money as well, because a clip made from a flawed still is a clip you end up paying for twice.