Starting with placing the foetus on some mattress foam or polystyrene to have a base for the foetus to lie on.
Then make the clay wall as if the the sculpt is pressed down into it, but don't press it down, build it up until it covers half of the sculpt, with fine lines around the joint. To achieve this I used different dental tools, fine paintbrushes, make up sponges and water. Water is good since the foetus is sculpted in chavant so the water won't have any effect on it except cleaning the pottery clay away.
When the joint is neat you need some keys for the second half of the mould to hold on to, just make them with something round.
Then I put up another wall around the one I've just made, to create the side of the silicone mould.
Once all of this is done the foetus is ready to be moulded.