Remove any insects from the elderflower heads and cut off the stems. Place the elderflower heads in a large bowl.
Place lemon slices, sugar and water in a pot and bring to a boil. Pour the hot liquid over the elderflower heads and cover with a lid. Let it infuse for 12 hours on the kitchen counter.
Strain the liquid and pour it into sterilized bottles. The elderflower cordial will keep for 30-40 days in the fridge.