You can make small or big ones. It's basically burning wood in an oxygen starved environment, you're really burning the gases off of it. Take a 55 gal barrel, lay on its side and fill full of your chosen wood. Weld up 2 braces to get it a foot or so off the ground. At the top weld 2 pipes that go up about a foot or so, 2or 3 inch pipes. Elbow 90 to side the width of the barrel, then down, then underneath. Drill holes in pipe that is now underneath the barrel. Put some wood underneath the barrel and set ablaze. After about 10 minutes the steam from the wood inside the barrel will be driven off and the gases will ignite and you will have one hell of a roaring and dangerous situation if the pipe and holes are too small. The gases ignite and bake the wood inside creating even more gases creating a helluva cycle. Pyro dream situation. When it goes out wait a long time before opening it... The next day open and you have charcoal. Google pyrolosis and biochar. I make small camp stoves, can inside a can that are much smaller and safer to start with. I've made rocket stoves and such. Fire brick pizza ovens. I am now tackling the mastery of masonry heaters. I get ceramic fiber, firebrick, satanite, and refractory cement from harbison walker in Knoxville. I've always been interested in the efficient creation of heat. So sorry for the novel. Biochar will give your garden one helluva kick in the ass as well
Something similar to what I was describing, not safe having one outlet IMO in case it gummed up.
Finally, I would suggest subscribing to jw934 channel on YouTube. He's a jap who has some genius ideas for simple stoves, many of which I've built. I'd try the can inside a can, you just drill some holes in the right locations you could have one burning at your house in 15 min
One of his:
http://m.youtube.com/v/xU6zkhbo_gU
The one I've tinkered with
http://m.youtube.com/v/6XxL6pPGGCE