Place the elderflower blossoms and lemon slices into a large pot or heat proof jar.
n another saucepan, bring the water, sugar and citric acid just to a boil. Make sure all the sugar has dissolved.
Now pour the hot syrup slowly over the elderflower blossoms and lemon slices. Stir to combine, and let it come to room temperature.
Now place an air tight lid on your vessel (if you pot lid isn't air tight, use plastic wrap or waxed wrap first, then place the lid on top.)
Let the mixture sit at room temperature for 48 hrs, stirring occasionally.After 48 hrs, strain the mixture through cheesecloth.
The syrup will be a slightly yellow to light amber color.You can keep the syrup, in an airtight container, in the fridge for several weeks as is, or you can proceed to can it, which makes it shelf stable.