Last year, I'd read an interesting bunch of material on "terra preta", biochar and just general charcoal benefits for plants. So I bought a bag of real charcoal pieces (not briquets) from a local hardware store to play around with. I powdered some and put it around some iris seedlings in the basement that had chronically rotting roots. The change was pretty impressive! No more rotting and those seedlings finally built up some size. I dug some into the planting holes for two hardy kiwis I was planting because I'd always had problems with root rot of these too. It's a year later now and those kiwis are doing great. Keep in mind I've killed probaby six others over the years.
In the meantime, while using the storebought charcoal, I'd gotten a gift of two metal barrels (one slightly smaller to fit inside the other) and made a barrel within barrel charcoal retort following plans I'd found on-line. I made 3 or 4 batches of charcoal that way and have been using it around various plants. But... I think I may have found a way that takes less effort and can do larger batches. I just made a makeshift firepit out of cinder blocks. Here's the third batch I've made this way. The general idea is to build a fire and then when it gets going good reverse what you'd normally do and put on smaller and smaller stuff until you get to twigs. Then when the coals look to be at an optimum state (lots of coals relative to ashes) dump a bucket or two of water on it to quench all combustion. The ashes mostly get flushed away with the water leaving a big pile of charcoal. I finally got the brush pile taken care of that had been sitting out back since last October's freak snowstorm hammered the local trees; and got lots of "free" charcoal to experiment with. You can't tell very well from the pictures but this charcoal is about three inches (~7cm?) deep in the center.