Cut the lemons into slices. Make a syrup with the water, citric acid and sugar and then cool. Put the lemon slices and elderflowers in a jar, pour the syrup over them, close the jar and leave it in a warm place for 5 days. After this time, drain the flowers and lemon slices and bottle the finished elderberry syrup and keep it cooled. Stored in a cool place, the syrup will keep for a good half a year.