How to make bio-char (charcoal) in a wood stove

Probably not something very many people need to make these days but certainly worth knowing how to do. I use charcoal as a soil amendment to my garden as well as a water filter for my aquaponics system. I make a few batches of charcoal throughout the winter and it's enough to make my own potting soil, supplement my aquaponics tank, make polishing compound and make a few marking tools.