If using fresh elderflowers, wash them and pat them dry.
In a medium saucepan over medium heat, bring the elderflowers and water to a simmer. Reduce the heat to medium-low and cook for 20 minutes. The liquid will reduce.
Place a fine-mesh strainer over a large bowl and strain the tea mixture to yield a smooth, refined liquid. Strain again if any seeds or floral bits come through. Discard the spent elderflowers.
Measure the hot liquid and add an equal amount of sugar. (Alternatively, measure 1 cup of liquid and 1 cup of syrup.) Stir until dissolved.
Let cool completely, then funnel the elderflower syrup into a jar or bottle. Use immediately or store in the fridge for up to one month.