The Necronomicon speaks of Simulacra as creations of Chorts, a race of inhumanly evil, shapeshifting demons immensely skilled in matter transmutation, worshippers of the dark deity YOG-SOTOTH. These contraptions were gifted to dark sorcerers in exchange for human sacrifices as means to achieve powers of Chorts, including immortality. Then goes a description how a sorceror connects to Simulacrum's brain and her soul is slowly and torturously transferred into this cast of cold, liquid metal. During this process, the sorceror subconsciously shapeshifts her new form into something more resembling herself - on the outside, that is.
To assemble a Simulacrum, we need Red Tower's lab.