Adapted from the Nettle Beer recipe (quantities of all but the elderflower are the same).
For 6 liters:
about 1 pint of elderflowers. Pick quite a lot, like about half a carrier bag, and then you have to strip the tiny flowers off the stems, which is a pain and takes a while, and you end up with about 1 pint of flower heads.
Boil 6L of water, turn off the heat, put the flowers in the water.
When it has cooled to blood temp (about 40 degrees), strain the flowers out and pour the liquid onto 750g of sugar. Stir til sugar is dissolved, adding the juice of one orange and one lemon.
When it has cooled to ‘room temp’ (MUST be below 24 degrees), add one packet of ale yeast (buy from homebrew supply shop or internet) and 1 teaspoon of yeast nutrient (optional, but that’s how we got these results), and 30g of cream of tartar.
Leave in the fermenting bucket for (about) 3 days, then transfer to demijohns with airlocks and continue to ferment until it has fermented out – you can check this with a hydrometer, or when it has stopped bubbling. Then siphon into bottles – you can prime the bottles with 1 teaspoon of sugar per L in each bottle which will help it to fizz – and seal.
OG: 1.048
A Storm Brewing 
