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.