Char Cloth is basically a piece of cloth that is charred, usually use with magnifying glass and sun to create amber.
To make Char Cloth, just put a piece of cloth into a small metal container and throw it into your campfire. Make sure the container doesn't let the fire burn the cloth,. You have to close it tight but not air tight as doing so will cause your container to explode (gas expansion).
After you end your campfire u can take out the container (might still be hot). Open it and you should see your cloth become black. It is now ready to use.
On a side note - if u place sticks into the container and do the same, you can get it charred enough to write like a pencil. Or u can simply "write" using Char Cloth, talk about leaving a message!
Hope this knowledge comes in handy for you...