For the readymades project we created moulds using a material called vinamold. This is a meltable vinyl material which is used to create flexible moulds.
I’m sure there are many different processes but for creating moulds for our found objects, we first made a clay wall around the objects, leaving a gap of around 1cm and being particularly careful not to leave any gaps the solution could leak out of. Then the heated solution was poured in and left overnight to dry.
In the morning once the clay was removed we were left with flexible rubber moulds, from which could be made casts of wax, plaster, chocolate, ice or really any material that could be cast as a liquid.