In the case of the Citroen soap model, i output the cad file to an objet 3d printer to make my first part.
Because of the viscosity of the soap and the fact that this is unpressurized molding it is not necessary to do any post printing cleanup or sanding on the 3d printed model, a mold can be taken directly from the print. This may not be true if you are using a 3d printer with a lesser resolution.
I first make a single cavity mold from the print and then replicate it a few times in smooth cast resin.
I'll then use the multiple resin pieces to create a multi-cavity soap mold to build in much higher efficiency during my molding process.