A simple way to make small amounts of charcoal would be to put a metal bucket of water next to your normal campfire. Once you have a good bed of coals built up in that campfire, shovel some out and dump them in the can/pail of water. Later, dump your bucket out on a screen or somewhere to the side, and let your new "charcoal" dry out. Just be sure the coals in your campfire are full glowing coals with no or very little flame.
Another way to make small batches is to use something like a 5 gallon metal paint/grease pail with the metal lid. Loosely pack the pail full of wood chunks and put the lid back on. It does help to punch/drill a few air holes in it. Then set that pail on a regular campfire. Just like making charcloth in a tin, smoke will start coming out of your pail throught those holes and around the rim of the lid. When the smoke coming out stops or slows way down, then set your pail off to the side to cool. Plugging any holes will help, and setting it lid side down with a little dirt scraped up around the pail helps seal that up from air.
Most methods involve burning your wood part ways down to just coals left, and then smothering out the fire. Part of the wood to make your future charcoal is burned up to provide the heat to convert the rest into charcoal. The "sealed drum" method uses an outside fire to "bake" your wood inside the drum without any oxygen. This drives off all that smoke - volatile gasses and moisture - leaving just the carbon fuel left. It's the same process as baking up charcloth in a tin. The "sealed drum" method leaves you with more charcoal inside your container, but takes more wood outside to provide the heat to bake the drum. And it is a little less -- messy.
Around here, Menards carries those big bags of Cowboy brand lump charcoal. Wally world occasionally carries some. But more people are starting to use the lump charcoal for their bar-b-qing, so more places are stocking it.
There is a Japanese "national living treasure" crafstman that only used softwood charcoal when he forges up saw blades. Each saw that he makes sells for 5K to 10K!!! But he is using all the old methods - including starting with wrought iron. He uses softwood charcoal because that is how he learned.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands