To do this I use chavant clay, and sculpt a thin layer on top of the Fimo. I have made an armature to hold the foetus while I sculpt, so I can reach it from all directions. The armature is bendable which allows me to move it around as I want.
It is tricky to sculpt this small and takes considerably longer time than what you can imagine.
To make the texture I used the tip of a pin, slowly working over the surface. I want the surface to be rough as if feathers would grow there one day.
When I was pleased with the sculpt it is time to prepare for moulding.