SETUP
Skill: Student should be able to complete a single turn in both directions on easy terrain. Can be done in a wedge or parallel and drill will work in both.
Terrain: for beginners find an easy sloping area that a student could make at least 3 turns across slope before having to change direction.
Snow: firm packed snow is best. Ice can work but decrease slope. Soft snow is very difficult for student to ROTATE the skis but can still work. You may need to have slightly steeper slope.
Safety: as the student will be traversing the width of the run, the big concern is traffic for above. Quiet runs are safer and using a second instructor above the student as a blocker can be effect measures.
HOW TO TEACH
Thinker (say): This drill is going to help you use the edges of your skis to get more grip and steering effort. Start across fall line in an athletic stance, flatten skis while turning legs slightly downhill until you start to move. Next turn legs uphill and get balanced on the uphill edges of both skis. Repeat until you cover the entire length of run.
Feeler (say): you are doing it correctly when balanced on the edges of the skis and you can feel pressure along the insides of your downhill foot and a lighter pressure on the outside of your uphill foot.
Watcher (show): perform the demonstration or show below video.
Doers (coach): while the student is trying this drill be sure to watch if you can see the bottom of both skis equally.
In a good result you will easily be able to see the logos on the bottom of the skis if in parallel. If in a wedge, you will only see the downhill ski base.