A whole bunch of notes:

1. No! That’s not a typo. It correctly reads 750g water. My preference is to measure liquids by weight (rather than volume) for accuracy.
2. Granulated sugar is regular sugar or table sugar. Slowly dissolving the large crystals of granulated sugar produces a beautifully clear syrup. Caster sugar (or superfine sugar) can also be used.
3. If possible, use an unwaxed lemon for the best flavour and clearest cordial. If using a regular lemon, scrub the skin in some warm water to remove any wax residue before use. OR throw caution to the wind and just slice and use the lemon without washing… I admit nothing!
4. Use 8-10 heads of elderflower, each of approximately 10cm in diameter. (I count two small elderflower heads as “one”, if together they measure about 10cm in diameter, across the top. Correspondingly, if I have a very large elderflower head (e.g. 30cm in diameter), I’ll count that as three heads.) Don’t wash or rinse the elderflowers – simply remove any large bugs/spiders by hand when picking. Washing the elderflower will just wash away the pollen (and hence the flavour). Any small bugs hiding on the stems will be filtered out when you strain the cordial.
5. Citric acid is a powder made from natural fruit acids. It acts as a preserver *and* adds greatly to the flavour of the cordial. It is an ingredient in things like sherbet and adds a pleasant “zing” to the cordial. It’s widely available in chemists/pharmacies, health food stores, home brewing suppliers and online. Make sure to get food or pharmaceutical grade citric acid. Try Home Brew West (Irish supplier) or Home Brew Shop (UK supplier) or Walmart or Bulk Apothecary (US suppliers) or, of course, Amazon! (Note that none of these links are affiliate links. I always purchase citric acid in my local pharmacy. Do shop around to get the best price!) In Ireland (June 2017), a small tub should cost €2 – €3. You can, however, leave out the citric acid if you prefer. Without the citric acid, you rely on the concentrated sugar solution to preserve the cordial. This should be fine for at least 6 months.
6. Don’t allow the cordial to infuse for more than 48 hours – after this time, the lemon rind can start to give a bitter taste to the cordial.
7. Sterilising glass bottles/jars is very straightforward. It is simply a way of thoroughly cleaning the bottles/jars to eliminate any bacteria etc which could spoil your cordial. To sterilise your glass bottles/jars, simply run them through a hot wash in the dishwasher. OR wash the bottles/jars in hot, soapy water and place on a baking tray. Put the tray in a cold oven and set the heat to 110C (230F/Gas Mark ¹/₄). Bake the jars in the oven for about 30 minutes to sterilise them.
8. Don’t squeeze the cordial through the cloth when straining – that will make the cordial cloudy. (That said, a cloudy cordial is not the end of the world! It will still be perfectly drinkable, but a beautiful, crystal clear cordial is my preference.)