Here is the tin can method: http://thedutchhermit.blogspot.nl/2012/12/making-charcloth.html
For this method we need 100% cotton cut into strips or small squares, a sheet of aluminium foil and some type of open flame such as a small outdoor fire or (as in my case) a kitchen stove..
The cotton is place in the centre of the aluminium sheet and you wrap it up into a nice package. On the top you make a small hole in the middle for the gasses to escape. Then it is placed in the fire.
It should look like this, Someone should clean that .... up!
I noticed that this method is way faster then charring it in a tin.. after less then 10 minutes on a low fire it stopped smoking, opened the package and the cotton was all black.. not burned but charred, nice!
Just in case you don't know what charcloth is used for.. It's used in order to catch a spark from a magnesium stick, flint and steel, firesteel etc. After it catches a spark you put it in a tinderbundle so you can blow it into a fire.