Take the sculpt lay on its back. This is messy, so lay down some protection for your surfaces!

Using your clay, you want to split your sculpt in half by building a wall around it (shown in blue). Then you can either use something round like marbles, or sculpt round blips into the wall to create registration marks to help seal the mold at a later stage (blue circles) do not let these be too close to sculpt or wall edge.

Next sculpt in some prying holes, these are thicker more square shapes that are on the outer edge of the wall (shown in orange) to help you open the mold easier at a later stage.

Next mix up some molding silicone and pour/brush over the first half of the sculpt. Leave this to dry, then keep adding silicone until it is about 1cm thickness.

Once completely dry, remove the clay wall. Brush on a somewhat thick layer of vaseline over the silicone wall that remains. Then pour/ brush on the second half of the silicone jacket.

Lastly, using Mod Roc create a "jacket" over the silicone. I used around 3 layers of mod roc, this will enure the silicone keeps its shape.