In a large saucepan, combine the water and sugar. Heat over medium-high heat, stirring occasionally, until all the sugar has dissolved.
Once the sugar has dissolved, remove the saucepan from heat and let it cool for a few minutes.
Meanwhile, gently rinse the elderflower heads and inspect for any bugs or debris. Make sure to remove any green parts as they can make the cordial bitter.
In a large mixing bowl, place the elderflower heads and lemon slices.
Pour the slightly cooled sugar water over the elderflower heads and lemon slices. If using, add the citric acid to the bowl and stir.
Cover the bowl with a lid or plastic wrap and let it steep at room temperature for 24 hours.
After 24 hours, strain the liquid through a fine mesh strainer or cheesecloth into a clean mixing bowl, pressing lightly to release all the liquid.
Pour the cordial into glass bottles or jars and store in the refrigerator for up to 2 weeks.