There are two ways to create a metal skeleton. The first method is the most common, yet brittle and weak. The second is unknown by most. For the first method, full skeletal remains and a large portion of the chosen metal are required. Then, either through magic or smelting, the bones are covered in the metal. The mage can now reanimate the bones to do his or her will.

The second method requires a live human or beast, and a small sampling of the chosen metal. Cast a spell to turn the creature’s bones into the metal, and burn off the being’s flesh and retrieve it’s remains for reanimation.
— "Grimoire of Condemned Spells", unknown author