Here, Krupp (yes, the same guys who made the STUG and Panzer III tanks) has a photo showing the process to curious Germans. The photo was recovered by the allies after the war, and here it is.
The process was essentially a series of molds the steel passed through, allowing it to be systematically shaped without weakening the metal or damaging it during the process. The shape made it more complex than American, British, and Soviet steel helmets, but all were made using a similar principle.
An interesting bit of trivia on an often overlooked facet of the World Wars.
