The process involves using instant coffee, washing soda crystals, a developing tank, water for the developing process, and a standard photographic fixer to finish it off. The procedure really isn't that much different from the normal negative development process, with the addition of the crystals (and, of course, the coffee). Apparently the negatives turn out "Slightly milky looking and brownish (due to staining action of developer) and also quite low contrast but certainly printable.
Need proof? The above image was prepared with coffee.