To make custom keycaps, I first sculpt a design out of a polymer clay, such as sculpey. Once I have completed the master design, it is then made into a 2 part silicone mold.
After the mold is finished, each cap is made by pouring a 2 part resin into to mold an letting it cure for the necessary amount of time.
Many of my caps are 2, 3, or even 4 shot meaning one color resin is poured and after it cures, another color is mixed and poured over it resulting in a multi-colored long lasting plastic keycap.