Obtain a solid carbon and graphite core, measuring 30-50 pounds total. The solid mass should have a very tiny diamond "seed" embedded inside of it, so that the carbon atoms can attach to it and crystalize when pressure begins to break down the solid carbon material.
Enclose the carbon core in an industrial-sized pressure cooker and heat it to 1,500 Fahrenheit, with a composite pressure of 850,000 lbs. per square inch. Retain this heat and pressure for at least 96 hours. This heat and pressure equal the forces placed on carbon below Earth's surface, where diamonds form naturally.
Turn off the pressure cooker after four days and allow the machine and material inside to cool. Once the cooker can be safely opened, a small amount of crystallized carbon will be found inside, theoretically identical to that of a naturally occurring diamond.