Clean elderflower, rinse quickly and pat to dry. Place in a big bowl or jug.
Slice lemons and add into the jug, then pour 2 L lukewarm water (water should be enough to cover the flowers!).
Cover and leave at least overnight or two.
Strain liquid and discard flowers and lemon.
Mix sugar syrup and strained elderflower liquid, then stir in 1 envelope (10 g) of citric acid.
Store in the fridge.
To serve add ice water and or soda.