Draw a large square on a piece of graph paper. Determine the length of the square's edges by counting the number of boxes from one corner to the next.
Divide the length in half to find the location of the midpoints of each edge. Then locate the center of the square by counting along the length of one edge to the midpoint, and then counting down an equal distance into the inside of the square. Mark the midpoints and center with small dots on the paper.
Draw a curved line between one midpoint and the center point. Continue the line from the center point, curving toward the other side of the square and ending at the opposite midpoint. The sinusoidal line should look like a wave, with one crest and one trough, each with the same distance from the middle of the square to the peak of the curve.
Cut the square out of the graph paper using scissors. Cut along the drawn sinusoidal line to separate the two halves of the square.