Teach them which muscles to use to keep their shoulders aligned with their hips, Have them stand, one at a time, with their feet slighly farther than shoulder width apart. Gently grab them at/about their shoulders and try to turn their shoulders out of alignment with their hips and feet. Do this in both directions.
The idea is to get them to have a physical awareness of the muscles used to maintain alignment, to remember the sensation those muscles had within to hold alignment, and to get on the beam/floor and re-create that sensation during the arm sweep and lift into releve', and maintain the tension that creates the sensation through the remainder of the turn. If it wobbles, maintain the tension and forget the wobble, because they'll fall anyway, and keep falling, and falling, and........
Make the "sensation" the only thing that counts as a success.... nothing else matters...... absolutely nothing else matters until they can consistently hold their alignment..... the arm sweep doesn't count, the releve' has no value, and spotting to mark the two phases of the turn won't help. None of it helps if they can't hold alignment, because every turn will become a "Frankenturn" that does whatever it wants no matter how much the person creating it protests.