For a CNC Lathe:
One takes a 3D file, such as an AutoCAD file, and translates it into CNC code, such that the machine can understand the input. The stock is then appropriately affixed and the code allowed to go to work.
For Hand Lathing:
An individual skilled in machining parts takes 2D drawings of the individual faces of the part and removes material from the stock in appropriate places using specified tools and by varying the speed of rotation. It is vital to have a plan of how to approach the part before machining it, as per the old carpenter's maxim 'measure twice, cut once' in order to avoid complications in manufacture.
To Outsource:
One takes 2D drawings of the individual faces of the part to a machinist, who will give a quote and an estimate of the time it will take to machine the part. The machinist will then follow the procedure above and produce, at the appointed time, a part matching the specifications required. At CMU the person to talk to is Jim Dillinger, who is in charge of the Student Machine Shop.