Cut a perfect circle by folding and unfolding. Fold a square of parchment into quarters and then into eighths or sixteenths to make a triangle. Measure the exact size of the cake pan by holding the point of the triangle at the center of the inverted pan. Using scissors, snip the edges of the paper to match the size of the pan, giving it a slightly rounded shape. Unfold the paper and press it smoothly into the base of the lightly greased pan.
Draw a pencil outline for square and rectangular baking pans. Hold the baking pan squarely in place on a piece of parchment with one hand, and use the other to outline the base of the pan with a pencil. Take away the pan and cut just inside the pencil lines. Lightly grease the pan, lay the parchment in it, and smooth out any creases.