Here’s a quick method for a small-scale production of biochar from the this Spokesman-Review article.
Making your own Biochar is not difficult, but it does take some planning. On a small garden scale, gather dried plant debris, small sticks from pruning work or dried animal manure. Dig a trench deep and wide enough to hold the material and pack it into the hole. Set it on fire and burn it until the smoke turns from white to gray and thins. Now cover the trench with an inch of soil to block the air from reaching the fire. When all the material has been reduced to charcoal, apply water to put the fire out. Dig out the charcoal and break it into small pieces.