Dissolve the sugar in hot water, then allow to cool a little. Once the sugar water has cooled, stir in the citric acid.
Cut the lemons into thin slices. Wash and pluck the mint leaves. Gently clean the elderflowers. Put everything in a suitable jar and pour the sugar-citric acid water over it. Leave it closed in a sunny place for 4-8 days.
Then bottle and store the syrup in a cool and dark place.