I had never tried one of the pre-formed molds at my pottery class before so I recently had a go with a small jug mold.
To cast the mold you use liquid clay slip poured into the mold. The mold is made of plaster this is because it has to absorb the water from the clay in order for the clay to set and form the jug shape.
Skipping all the other details in between like filling the mold and finally removing the piece from the mold, here are my during and after glazing pictures.
It's not perfect but it does hold liquid and pour quite well.