Their creation requires the body of a deceased dragon. The body can be artificially developed within a very short period of time. The Necromancia will resemble how they looked in life, but with darker skin and with rotting and continuously regenerating flesh.
Both the requirements for, and the process to create, a Necromancia are unknown. However, it appears they require little in the way of equipment, and many can be done within a relatively short period of time.