(this sounds strange, but it's amazing. it doesn't just cure diarrhoea either, my dad used it on a gum infection and it worked better than the antibiotic the dentist had described)
grind some coffee, or buy some pre-ground. Don't use the instant coffee with the sugar and whitener already in it.
Fire up the barbecue or start a campfire
put a pot or frying pan on the barbecue or campfire, and pour the ground coffee into it.
Put on some work gloves or wet a cloth and snuff out any flare-ups that appear in the coffee grounds. Keep the coffee grounds on heat and stir once in a while until you have powdered coffee charcoal.
You can do it on lower heat and not watch the pan, and there's no harm in doing it indoors either, only it smells pretty bad while it's charring unless your home is very well ventilated.
Experiment a bit to find the proper dosage, there's no harm in eating a lot (it's just coffee after all) but a little goes a long way.
hope this help!