Collect elderflower umbels on a sunny day. Rinse with cold water and shake dry. Rinse oranges and lemons and rub dry with a cloth. Cut into slices.
In a saucepan, bring 2 liters of water and the sugar to a boil. Simmer until the sugar has dissolved.
Put the sugar water, orange and lemon slices and the elderflower umbels in a bowl or in a glass or clay bowl. Now add the citric acid.
Cover the bowl with a cloth, leaving a small gap so that the syrup can “breathe”. Let the syrup stand for 3 days.
Then line a sieve with a cotton cloth and filter the syrup.
Pour the elderflower syrup into carefully cleaned bottles and seal them airtight. Store in a cool and dark place.