Peel the ginger and then slice or shave it as thinly as possible. A mandolin with a guard is the best thing to use.
Place the water, sugar and rice vinegar in a saucepan and bring to the boil. Add the ginger and reduce to a simmer. Cook for 5 minutes and then allow to cool thoroughly.
Once cooled, keep the ginger and the cooking liquid in the fridge until needed. The sugar and the vinegar will both act as preservatives so it should last a good long while!
«The easiest way to peel ginger is to use the side of a teaspoon. Just scrape off the thin skin and you'll waste a lot less of the ginger than when using a peeler or a knife.»