You can look at this clip to see how to make your own coaler. it basically is a can inside a can. The inner can only has a few holes for gas to escape and the lid gets put back on lightly, is stand on the bottom of the larger can, which has a few holes drilled around the base to let oxygen up between the sides of the two cans. This space is packed tightly with pieces of wood and twigs then set alight on top, it will burn to the bottom and extinguish itself. The inner can is to prevent oxygen getting to the material being reduced to carbon. This gives black coals with no ash.
This stuff is amazing and a game changer for the future of our species. Large populations lived on some of the worst soils possible using this method. Once the carbon is loaded with nutrients it has a half-life from
1000 years plus..... It is the gift that keeps on giving. Another part of what makes it work is that microorganisms can be safely harboured inside. You can kill your carbonated soil by using salt based fertilisers, but in a year or so, it will bounce back. SO once you start, treat that soil as a living organism.