Whack in the sugar and pour over a litre of boiling water, stirring until the sugar has fully dissolved. Add the rest of the water, the acid and yeast nutrient. If you wish, take a hydrometer reading to see how strong it is. The campden tablet (if using) should also be added now – this kills bacteria but can cause headaches in some. If you do use a campden tablet, leave the mix for 24 hours.