Now we have our can ready. To make char, take the cotton cloth and cut into approximately 2 x 2 inch squares. Place eight of these, stacked on top of each other, in the metal container and tightly close the lid with the hole in it. Place the container upright on a hot plate and set the hot plate to 'HIGH'. The hole in the container will release the smoke and pressure in the can as well as limit the oxygen in the can so that the cotton will carbonize but not burn.
Many people say to let the cotton cook until the smoke quits coming out of the hole.
I've found that if left that long the cotton turns to brittle charcoal. Good char will be black and carbonized, but still flexible. It usually takes about five minutes to obtain good char in my paint can if I start with a cold hot plate. Over cooking is easy to do, so you might want to experiment. After you take the can off the hot plate, DO NOT open it until the can is completely cool.