Fold a round filter paper in half, then in half again. The filter should now look like a quarter piece of pie.
Open the last fold so the filter looks like half a pie. Lay the filter on a solid surface with the folded edge toward you. the center crease should be facing downward.
Place your left thumb just to the left of the center point where the crease meets the folded edge. Grab the right corner of the folded edge and fold it upward so the right half of the folded edge lines up with the center crease. Do the same on the left side. The filter should look like a quarter piece of pie again, but this time it has two flaps that meet in the middle.
Open the the flaps so the filter looks like half of a pie again. The filter should have three creases which look like three rays emitting from the center. Flip the filter over, keeping the folded edge toward you. Now, all three creases should be pointing upward.
Make four downward creases, dividing each of the four regions between the upward creases in half. This is like folding a fan.
Unfold the filter. It should now have 16 sections with creases alternating up and down.