1. draw a tiny closed half circle, going vertically, in the center of the paper.
at the top of the vertical line where the circle halves, draw a very tiny circle, as well with the bottom end of the line.
2. draw the other half of this circle, mirroring the previous half. leave a space in between the halves. they must not touch.
3. repeat these steps, creating larger half circles that overlap the previous ones, with the tiny circles increasing in size too. the halves must continue to mirror one another, and the halves still cannot touch. the halves must overlap the previous halves no more than a 4th of the previous halves.
I think I would need to tinker with the element of the overlapping, since not everyone followed the idea of that remaining consistent. However, this was my element of ambiguity, which allowed various results to form. I enjoyed watching the participants follow the instructions and make it their own, composing it carefully and adding their own element to the guidelines.