Choose paper that is easy to fold.
Fold in half twice.
Fold a third time so that two adjacent sides are together in order for the point to be the center of the circle.
Clip a curve on either corner as long as it is not the center point.
Unfold the paper and use it as the pattern for something like card stock that is sturdy; elastic would be better.
Cut out to the pattern, including around the slits.
Glue the dull side of aluminum foil to the card stock or equivalent.
Tape the slit sections together.
Use a piece of white paper to make sure there is a focal point.
Exactness does not seem to matter!
I know this probably belongs somewhere else like Instructables, but I will put it there later!