Depending on the accessible materials they basically forge a tattoo gun and harvest ink.
Ink is most commonly made from soot. Pieces of plastic (from a chess or checkers set) are melted in a confined area, a piece of paper is held above the burning plastic to collect the soot. The soot is then scraped off and mixed with lotion or baby oil. Thus creating your ink.
Needles are made from the spring inside a pen. The spring is unraveled and sharpened, then wrapped with thread to hold ink.
The actually tattoo gun is made from beard trimmers (available to buy on commissary).
The motor is taken from the trimmers and attached to a makeshift bracket.
A hollow tube (from a pen most often) is attached to the bracket as well. The needle is guided through the tube and attached to the motor. When the motor is turned on it will rock the needle back and forth in the tube.
The last step (if available) is an A/C power adapter. This is used to control the intensity of the tattoo guns motor. Helpful with shading and general smoothness in the tattoo lines.
You are now ready for your prison tattoo. Find a private area and a lookout to alert you to the presence of a correctional officer. Sterilize your needle (using a lighter and wiping off carbon deposits) and your masterpiece begins.
– Christopher Isani