Bring the sugar, citric acid and 1 1⁄2 l water to the boil and simmer until the sugar has dissolved.
Prepare elderflower umbels. Wash lemons hot, grate dry and cut into slices. Pour the sugar syrup over the elderflowers and lemon slices. Leave to cool, cover and let stand for 3 days.
Pour the syrup through a gauze cloth and fill into bottles. Close bottles well and put them in a cool place. Fill up the syrup with sparkling wine or mineral water in a ratio of 1:4. Cool as desired with raspberry ice cubes.