I built a mould to make elastic prototypes of my flashlight holder. It gave me a good picture of the manufacturing process.
As a youngster I moulded small tin figurines to play with. Well, in lack of money I mostly melted lead bullets I got for free from the local rifle club.
An engineer friend of mine demonstrated how he made silicone moulds for his machine parts. I could clearly do it the other way around and pour soft hobby silicone into a home made mould and make elastic prototypes.
Eventually I managed to make a functional plaster mould to pour my silicone solution into. There were lots of problems getting the silicone out of the mould without ruining it. The blue colour you see inside is glossy paint to protect the plaster. Otherwise the solidified silicone sticks to the plaster breaks it apart. There was a lot of trial and error before I got reasonable results from this one.