I have postponed it because I know that while the duck is easy to make, it will be hard to explain. But let me try anyway. I know that many readers want very detailed instructions, so here is my recipe in all the dreadful details. This is where it starts to get complicated, but if you follow the figures you should be able to understand my instructions. Please read all the way through the instructions once before doing anything else.
Lets start with the template for the two sides.
We will start with three construction lines. Divide the cardboard by a horizontal line on the long edge of the cardboard. Mark two points 100 mm over and under the horizontal construction line on the right vertical construction line. On the left vertical construction line the two points are 40 mm over and under the horizontal lines. Using the points you just marked as centres, draw two circles with radius 315 mm on the right construction line and two circles with radius 170 mm on the left construction lines.