Place a sheet of craft paper over the light switch that will covered by the switch plate cover. Draw the exact size of the cover, carefully marking areas for screws, switches and outlets with a pencil. Cut out the drawing to use as a template.
Divide the polymer clay into 12 equal-size pieces. Roll each piece of clay into a long snake shape that is the diameter of a pencil.
Rip off a sheet of parchment paper and place it on the cutting board. Place the clay snakes on the parchment paper, aligning them lengthwise next to each other.
Lightly roll the snakes flatter with the rolling pin to a thickness of 1/4 inch, making sure the lines between the snakes are visible.
Place the craft paper template over the clay and cut out the template and all the openings with the craft knife. Use a wooden skewer to make the screw holes.
Pick up the parchment paper carefully with your hands and place it on the cookie sheet. Bake the switch plate in a 175-degree-Fahrenheit oven for 20 minutes. Remove the cookie sheet from the oven and let the completed beadboard switch plate cool for 20 minutes.