Take another piece of cardboard and make a copy of the side template including the horizontal construction line, but excluding the tail, which will have to be different on the belly. 50 mm extra for the seam. Now you need to transfer the line A'-B to the belly template. Draw a line on the belly template along the side template. Repeat on the other side, and the tail on the belly template is done.
Next template is the wing .
The wing should follow the ducks back when it flies. Make a copy of the back-line on the side template on a new piece of cardboard. The size of the wing is up to you. Mine is app. The lower edge of the wing is unimportant, use your imagination.