Place the elderflower heads (each is about the size of a hand) into a large bowl and cover with 16 cups of water. Leave for 24h.
The following day add 16 cups of sugar and 1 tablespoon of citric acid to the flowers in water. Leave for 24h, but stir well a few times during the day.
On the third day, strain the juice through a colander into a different bowl. Once you've strained the liquid, also squeeze flower heads really well to get all the liquid out of them. Discard used flower heads, add the remaining citric acid and a juice from one lemon to the syrup. (Here you can taste the syrup to see whether the acidity versus sugar combination is to your taste. Based on this, decide how much citric acid and lemon juice to add.) Stir really well and let sit for one hour.
Heat oven to 250F, and wash about 4-5 glass bottles (each of 1 liter or 4 cups in volume) in soapy water. Rinse well, and pat dry. Place on a thin pan and place in the oven to dry completely thus sterilizing them in the process, for about 15 minutes.
Carefully take the bottles out of the oven, and strain the syrup from the bowl into the warm bottles. This will be the second time you'll be straining the syrup, so this time use a gauze (or a tea strainer) to strain the syrup through.
Leave bottles in cool place. The syrup juice can last up for a few weeks if held in a cool place (or fridge).
Once ready to serve, put three or four tablespoons of the syrup into a glass and top with regular or mineral water.
If you want a smaller volume of juice, halve the ingredients. The process stays the same.