I learned to make char cloth like every body else: Put your natural fiber cloth in a can with a small hole punched in the top, set it in the fire, watch until the smoke stops coming out of the hole, then set it aside to cool. If you do it right, you'll open the can to find uniformly charred material that will catch a spark from your flint and steel readily. From this you can coax it into a beautiful flame.
While there are many references in period journals to starting fire, few ever give any details. There is no mention of charring material to catch a spark, and no mention of a can with holes in it to do that. We just don't know for sure what was used, or how it was prepared. It has been a mystery, and we just followed the way everyone else did it.
For years I have carried a tin that included my char, striker, flint, and some other small fire starting helpers such as pieces of pine pitch and a candle stub. The top of my tin has a hole so that I can use it to make the char. When I am done with a batch of char I drip some wax from the candle stub over the hole to seal it up. This allows me to carry everything I need to start a fire as well as a method to replenish my char cloth.
My kit works great, but when looking at museum pieces I notice that historic tinder boxes do not seem to be designed to be placed in a fire. So how would they have made char? Or what would I do if I lost or damaged my char making tin?
Recently I was at a camp demonstrating how to make moccasins using some cheap cotton canvas for a test pattern. When I was done I had some leftover scraps, and being a frugal mountaineer I figured I would make it into char. Since I was already sitting next to the fire, and my tin was out of reach, I decided to try making it without the tin.
I dug a small hole in the ashes, laid in the cloth, and covered it back up. I moved some of the fire on top of where the cloth was and left it. A little while later I poked in the ashes and saw that my material was perfectly charred. Of course, taking it out of the still hot fire caused it to ignite immediately and I had to stomp it out. Even after stomping on it, it was some of the best feeling char I had ever made. It caught a spark great, and was still very soft and flexible. I am sure I am not the first man to think of this, but my skeptical camp mates were impressed that it worked. For all we know, this may have been how it was done historically.
The principal for why it worked was the same as for the can with holes in the top: Being covered with ash does not allow any oxygen to get the cloth so it chars but doesn't burn. Sand or dirt over the material to char will also work.
I continued experimenting with the method. The biggest drawback I could think of was that when I took it out of the hot fire, it would ignite and I'd have to stomp it out. What I've come to prefer is putting the char in the ashes in the evening and removing them in the morning when the fire has burnt out. Unlike how char cloth can get brittle if left in a tin too long or if it gets too hot, leaving char in the ashes overnight does not seem to have any such disadvantage -- it still comes out great.
This method will also work for other materials such as certain types of rotten punky wood. I have had the best luck with rotten cottonwood or some aspen. One time I was out of char and pulled an old piece of charcoal out of a firepit from under the ash. It worked for the same reason. As with most things, there is no one right way to do it. The trick is to learn to improvise and make do with what is available.
So I urge you to think outside the box. The next time you are out on the ground, give this method a try. Or try and come up with other methods. Imagine yourself in a situation where you are without char and need to either make some more without a can, or make do without. If you shoot a flintlock don't forget that it's more than just a weapon, it is a tool with many uses and has everything you need to start a fire.