Boil water with sugar for 10 minutes to a syrup.
Leave to cool.
Cut lemon in slices and place with elderflowers in a jar. (1,5 liter e.g a cornichon jar)
Pour syrup in the jar and close it.
Place in fridge for 2 - 3 days.
Drain in a pot (use a cloth) and bring to boil.
Rinse bottle in very hot water, fill in the syrup and close immediately. (1 liter)