Please, please, PLEASE be sure you're picking the right flowers - I don't want anyone making a lovely cocktail of Deadly Nightshade, or something...
Once you're sure you've found the right thing, you proceed as follows:
Pick 30 or so heads of elderflowers. Snip them from the main stalk. Scrub and slice 2 lemons and add to flowers, 2oz/50g citric acid (can be ordered online), and 2kg sugar in a big bowl you won't need for a few days. Bring 2 1/2 pints of water to boil and pour over the mixture. Stir until the sugar has dissolved, cover with a cloth or plate and leave for five days. Feel free to stir each day, if you're so inclined.
On day 5, Line a colander with a thin cloth (muslin is what we use) and set over a large mixing bowl. Tip the elderflower mix into the colander and let the cordial strain through to the bowl. Discard everything left behind.
You should store this in a sterilized glass bottle, but we're fresh out, so I poured it into a cleaned out plastic juice bottle, and we haven't perished (yet) Let the record show, I did say "sterilized glass bottle", though.
Since most Americans are unfamiliar with cordial, let me tell you right now - you don't drink this stuff straight! Blech. It's pretty strong - I put less than an inch in a tall glass and top it up with cold water. Another popular option is soda water. However you wanna roll...
Once you've made this, it'll change you. You'll turn into a tortured soul that always travels with scissors and a plastic bag, scouring roadsides and shouting, "There! In that gully! There's some there - can we stop?" while travelling 70 mph on the interstate. So don't say I didn't warn you.