Have thought about trying this but just haven't until just now... First attempt the recipe I used was >>>
1) Get a swatch of 100% cotton fabric (I used an old t-shirt but denim is a good choice as well. Any plant based fabric will work - cotton, linen, jute, etc...)
2) Wrap swatch as anaerobically as reasonably possible (by hand) in Reynolds wrap.
3) Cook in toaster oven on the highest temp setting (450 for my toaster oven) for 20 minutes.
4) The resulting char cloth is uniformly charred dark brown / black but is only mediocre in terms of function... not bad for a first attempt actually but definitely needs some improvement. I think that answer will be to just cook it longer. I really don't think that you can overcook it - I'll probably go for 1 hour at 450. One tip would be if you are going to use this method - it is probably a good idea to move the toaster oven outside (porch or patio). A 3" x 1" square gave an interest fragrance to my kitchen.