Add the lemon slices and elderflower blossoms to a large heat-safe pot or large jar with a lid that's at least 2 litres (8 cups) in size.
In a separate pot, add the sugar, water and citric acid and let the mixture come just to a boil (make sure all the sugar has dissolved).
Pour the sugar syrup slowly over the lemons and elderflower blossoms and stir well to combine.
Let the mixture come to room temperature and cover with a lid (make sure the lid is airtight, so use plastic wrap if necessary).
Let the mixture sit for 48 hours, stirring a couple of times each day.
After 48 hours, strain the mixture through a strainer and cheesecloth to remove any impurities. You'll be left with a clean-looking syrup that's slightly yellow in colour.
Add this syrup to a pot and bring it to a boil. Turn the heat down so the mixture sits at a low boil for about 10 minutes.
Add the hot syrup to prepared jars or bottles for canning (this syrup is shelf-stable if proper canning procedures are followed), or store in an airtight container in the fridge for up to 2 months.