This year I've made three litres of the stuff and it came out reasonably well, so I'm going to share the secret with you. Possibly a bit too late to go out and try it, sorry (my fault as it's a week since I bottled the finished product), but there may still be a few late-flowering branches to harvest.
There seem to be two schools of thought, as exemplified by the first two recipes I happened upon in my research. By far the most common seems to be that you make up a syrup with hot water and sugar and then pour it on to the flowers (or, sometimes: layer the flowers with sugar and pour hot water on it, thus dissolving the sugar and making the syrup). The other is to soak the flowers in plain water and then cook it up afterwards with the sugar. I rather like the cooking-afterwards idea from the point of view of making sure there's nothing nasty in it (some references also claim that the flowers contain a mildly poisonous alkaloid which is destroyed by cooking). But there are also other advantages: you can make it on any scale you like as you don't need to know in advance how much water and sugar to put in, and if it goes wrong you can junk it without having wasted a ton of sugar. The main disadvantage of doing it this way round is it's more liable to go off while the flowers are soaking because the sugar is not there to preserve it (this is what happened to my second batch last year — I still made the cordial and it was marginally drinkable but definitely far from perfect).
Recipes from the cook-before ideology tend to tell you to pour the water on to the flowers while still hot. This does not work in my experience (and an independent witness has also confirmed by experiment). When I made the first batch last year I intended to use "cooled boiled water", then thought it doesn't matter if I put it on a bit warm — and didn't wait nearly long enough for the kettle to cool so the water was hotter than I thought. When I poured it into the bowl, the first flowers that it touched instantly turned brown, followed in due course by the whole top layer of flowers. After two days' soaking the liquid was dark brown, and while it definitely tasted of elderflower it also tasted of burnt. I drank quite a bit of it, but in the end poured the last litre away.
Elderflower CordialPick the flowers on a dry warm day when they are in full bloom. Select only flowers that are white with bright yellow stamens and don't use flowers which aren't open yet. (I take the preceding instructions on trust from several web sites, but they seem sensible enough.) Shake them lightly to remove insects (but not so vigorously as to shake off the pollen, since at least one source claims that the flavour comes from the pollen). General advice seems to be not to wash the flowers, but do inspect them to make sure there's nothing too nasty. I may point out at this stage that you won't be able to get rid of all the tiny thunderflies from the flowers, but then this is why the cordial is going to be strained and pasteurised.
Cut off the white flower heads into a bowl. Leave as much green stem behind as possible, but it's not necessary to be so meticulous as to take all day over it. Add a sliced lemon and maybe also a generous squirt of lemon juice if you happen to have some handy. Add enough cold water to just cover all the flowers. A litre of water will cover very roughly 25 flower heads.
Cover the bowl and place in the refrigerator for 3–5 days, stirring occasionally. If there isn't space in your fridge, leave it for a maximum of 24 hours. The top layer of flowers will probably slightly discolour each day; when you stir it, it will be white on top again if you started off with enough flowers.
Pour the whole thing into a large saucepan and place on a gentle heat, stirring and observing to make sure the flowers don't turn brown. Heat it up to roughly the temperature at which water comes out of your hot tap — don't boil it.
When you feel it's had long enough, strain it through a folded muslin cloth into a measuring jug. It will be distinctly yellow (which seems to be mostly due to the pollen, and for this reason it may be slightly clouded). Squeeze out as much liquid as possible (then dispose of the remains on the compost).
Rinse out the pan, pour the strained liquid back into the pan and put it back on the gentle heat. Now add 750g of sugar for every litre of liquid and stir till it dissolves. Then add about 2 teaspoons of citric acid for every litre and stir it in. (Chemist shops should sell this although I've found that Boots don't. My source in Oxford is the pharmacy which is built into the East Oxford health centre at Manzil Way. If you don't have any then lemon juice would probably do, though I haven't tried it that way.) The function of the citric acid is mainly to improve the flavour and counteract the sweetness of the sugar, so it's important to taste the mixture to get the amount right (spoon a small amount into a glass and add cold water — don't sip it neat). Citric acid is also said to aid preservation.
Once this is done, you have basically finished. Take it off the heat when it looks like it is about to boil, then bottle it. When they are cool enough, put the bottle(s) in the fridge, maybe freezing one for later in the year. The cordial is supposed to keep for 2—3 months if kept refrigerated.