The basic idea behind charcoal is to take wood and drive off all the volatiles. That leaves behind pretty much pure carbon and a little ash (things like calcium, potassium, etc.). The way you drive off the volatiles is with heat, and you apply the heat without any oxygen in order to keep the wood from lighting on fire.
What this means is that if you want to make your own charcoal it is pretty easy. You need 1) wood, 2) some kind of closed metal container or pit to keep out the oxygen and 3) a source of heat. This article shows you the basic idea on a small scale:
If you want to make a lot, you can use the pit method:
If you want to make truckloads, you need a charcoal factory: