Cutting the cardboard sheet
First, cut a 14 x 28-inch (or 35 x 70- cm) strip and an 8 x 28-inch (or 20 x 70-cm) strip from the poster board. Fill red color on one side of the narrower strip, and in the bulb on the large strip.
Follow the measurements and draw a thermometer on the larger strip, showing different temperatures. After that, remove (or cut out) the 4 x 22-inch (10 x 55 cm) section above the bulb of thermometer.
The slit & strip
Now cut another 14 x 28-inch (35 x 70 cm) strip from the second sheet of the poster board. Cut another 100-inch (25-cm) slit that is about 4 inches (10 cm) from the shorter edge of the strip. Make sure it is at the center and in horizontal position.
Adjusting the Strip
Position the strip behind the thermometer with the borders of both strips taped together. Into the slit, insert a narrow paper strip in a way that the red side is visible through the thermometer’s cut out section.