For the elderberry juice, finely slice the lemons and oranges. Put the elderflowers in a large pot with the remaining ingredients.
Stir everything well until the sugar and citric acid have dissolved. Let stand for 5-6 days, stirring every day. Then remove the elderflowers, orange, and lemon slices from the pot and boil off the juice.
Pour through a sieve (to remove the remaining flowers) into bottles. Allow to cool and store the elderberry juice preferably in a cool, dark and dry place.