Each one of my students made a mark. I went around the room about two times. They copied the marks that their fellow students made. After they made a big mess then we began the process of pushing and pulling marks on the drawing surface.
I instructed them that there would come a point where they would not want to follow the main drawing and start to produce their own. Sure enough every student ended up with a different drawing. This was my finished piece.
I like the idea of starting somewhat at the same point and then letting the student find their own solution. It help them to begin to understand that everyone will solve the problem differently and that in art there are countless solutions.