Google 'tire molds' and you should find some pictures.
The mold is the negative shape of your part and a blob of uncured rubber large enough to slightly over fill the mold is put in and the mold closed. You need pinholes to let trapped air escape in the detail of the mold; on tires you see small round bits of rubber sticking up all over the tread, these are where the soft rubber entered the pinholes. Once the mold is locked closed it is heated by steam or electrically for several minutes to cure the rubber and then opened and the part taken out.
It is simple.
For small quantities of raw rubber you should find a company making rubber products and see if they will sell you a few pounds. The uncured rubber has to be kept in a deep freeze or it will slowly cure at room temperature.
An open mind is a virtue...so long as all the common sense has not leaked out.