Make a white area that covers what you want to hide. Cut a circular hole in it.
I have memories of having made a doughnut symbol... following instructions on the worldofo training site.
Yeah, I tried to follow otraining.net's instructions, but I must have been doing something wrong, as the result did not materialize. I only left myself a few minutes to implement the circular window before we left, so I settled for the ghetto rectangles.