I was first presented with the idea of “custom action figures” from ToyFare Magazine back in the 90’s. My first attempt was to turn my Masters Of The Universe King Randor (it will never be valuable) into Bishop from the X-Men. It turned out horrible, but I kept trying.
In the past I made my own figures and accessories using silicone molds and plastic resin, currently I use 3D printers. A lot if the time I will just use a store bought figure to transform into something new.