Be sure to pick elder flowers from a clean region, so you don't have to wash them before use. Crumble them n a deep pot, remove the larger stems. Pour in the water, add the sugar, citric acid and stir well.
Wash the lemons and cut them into very thin slices, along with their rinds, and add them to the liquid. Stir well again and rub the flowers lightly with your hands.
Leave them in the fridge for 24 hours, stirring occasionally. You can add additional sugar if you want it sweeter.
The next day, add the washed mint leaves and leave the mixture to sit for at least another few hours or another 24 hours.
Strain the mixture through a strainer or cheesecloth, distribute into glass bottles and put them in the fridge.
The juice is ready for direct consumption, serve cooled with ice cubes, a lemon slices and a sprig of fresh mint.
If you'd like to store it longer, distribute it into glass beer bottles, add 2-3 rice grains per bottle (to carbonate the drink) and sterilize for 10 min.