Pick your elderflowers and leave them outside on a tea towel (in the shade), so that any potential bugs crawl away. Don't wash them, as this would damage the pollen and the natural yeast bacteria.
In the large plastic container dissolve sugar in the cold water. Stir until it's properly dissolved and the water looks clear.
Remove any green stems from the elderflowers and add them in the container
Add the lemon zest, vinegar and the lemon slices. Stir slowly and loosely close with a lid.
Leave to ferment in a room temperature for 48 hrs and check regularly for any signs of fermentation/yeast activity. Your elderflowers mixture should start to bubble within 48 hrs. Open the lid and stir couple of times a day to introduce more new air bacteria.
If you don't see any bubbles within 48 hrs, you can add some champagne yeast. This is optional.
Pour the elderflower liquid through a muslin cloth (or a fine strainer) to a bottle or a fresh plastic container leaving about 5-6 cm (2 inch) from the top to allow for further fermentation. Loosely cover with a lid. Discard the elderflower heads.
Leave to ferment for another 3-4 days before bottling the elderflower champagne to smaller glass bottles leaving at least 3 cm gap from the lid/bottle stopper. Check every day to let any gas pressure out.
Leave for further 5-7 days to develop in the cold environment (or longer), before chilling in the fridge and enjoying your elderflower champagne with your summer picnic or garden party.