For best results, juice the ginger with a macerating (slow) juicer. Bring water to a boil together with other spices, simmer for 20 minutes, then add juices and sugar and allow to cool to at least 30 degrees C.
Add cooled wort to the trub left behind from a stout ferment, e.g. The Dark Crystal.
Open ferment (ideally in a food grade bucket with lid), no airlock, just cover the top of the bung with clean foil. Choose a warmer location and/or season, e.g. kitchen, aiming for 22-24 degrees C (but up to 30), or substitute a wide temperature range yeast (e.g. Loki).
Bottle reusing 8 empty 2L carbonated beverage bottles (no root beer bottles unless completely eradicating the persistent flavour extant in them).
Colour (and dark rum-like flavour) in the final beverage can be tweaked by substituting table sugar for demarara, brown sugar, or even molasses, heading towards a 'dark and stormy' cocktail finish. Alternatively, the palest option can be given an oceanic tint by adding Blue Curaçao before bottling.