First, I needed to make a ‘positive’. A ‘positive’ is the object you want to make a cast of, so my object is a cup. I made these two on a pottery wheel. They are solid forms so that when I pour the plaster over them, they won’t collapse.  

When they are ‘leather hard’ (that means they are half way between wet clay and dry clay), I can begin to make a mold. 

First, I turned the cup upside down in a bucket, then built some ‘form work’ (that is a wall for the mold) out of sheet metal (I used roof flashing. It’s cheap, from Bunnings). Then I mixed up some plaster and poured it over the cup.