Draw a straight line using a ruler and pencil. Measure the line and keep track of its length.
Place the center point of the protractor at one end of the line and align the protractor's flat base with the line you drew. Draw a dot at the point on the protractor's curve that indicates the interior angle of your desired polygon. The interior angles of some common regular polygons are as follows:
Triangle: 60 degrees
Square: 90 degrees
Pentagon: 108 degrees
Hexagon: 120 degrees
Octagon: 135 degrees
Regular polygon with n sides: (n - 2) * 180 / n
Line the ruler up between the end of the line and the mark you made to indicate the angle. Draw a line along it that is equal in length to the first line.
Repeat this process until you have drawn the entire polygon, each time aligning the protractor with the most recent line you drew. If a regular polygon is correctly constructed, all the angles and sides should be equal and the ends of the first and last lines you drew should meet at the same point.
Cut along the polygon's outline with a craft knife or scissors.