Carefully wash 150 g of elderflowers. Bring 1 L of water containing 1 kg of sugar to the boil, then leave to cool down. Then add the elderflowers and 25 g of citric acid to the sugar water. Leave in a dark, cool place for three days. Prepare the bottles in which to store the syrup. Pour the elderflower syrup through a sieve and then pour into the bottles. To conserve the syrup, add a few drops of alcohol (80% ABV) and then set alight.