There are two common ways to manufacture ghost guns. Anyone can go online and download a blueprint for a ghost gun and create the gun with their 3D printer. 3D printed guns are illegal under the Undetectable Firearms Act 18 U.S.C. §922(p). A 3D printed gun must have a metal plate inserted into the body to be legal. This plate will allow it be detected by a metal detector. Otherwise it will be illegal. If you do not own a 3D printer, a ghost gun can be created by using an unfinished receiver. Unfinished receivers can easily be purchased online or in stores.