So far, the sections that will comprise the shell are flat, and don't look shell-like at all. Each one needs to be curved round, into a 3D shape.
As a reminder, here's a single, flat, section:
As it was stiffened with a heavy Vilene it will curve quite nicely without bending. I formed it into a curve using my fingers and a wooden tool I found in a drawer - I have no idea what it's meant for, but it's a nice thing to use!
I tried to take my time over this to make sure that the final shape was even.
In the picture of the unbent section above, you might notice that there are two flat spots on its otherwise curved edges. I brought these round to meet each other:
And ladder stitched the edges together with a curved needle:
This gave the finished section:
With all nine of them done, they're ready to be assembled into the final piece: