Snip the flowers off the stalks into a quart Mason jar. Remember the stalks and leaves of elderberry plants are toxic, so snip off as much of the stems as you can. Getting them all is not possible, but spend some time removing the stems.
Cover the flowers with the alcohol and seal. What alcohol? Your choice, really. Typical is 80-proof vodka, but I prefer 100-proof vodka. And once a year I use Everclear or some other 151-plus proof alcohol. Why? The flavors and aromas of elderflowers are not all extractable by water, or alcohol for that matter. I find that the higher the alcohol content, the cleaner and purer the elderflower flavor. Of course, if you use Everclear, you will need to cut the liqueur with lots of water or ice -- otherwise it will knock you down in a hurry.
You will want to submerge the flowers completely in the alcohol. If you don't, the top layer of flowers will oxidize from contact with air, turning brown. This doesn't harm your liqueur, and for years I made it this way and it was fine. But a better way to do it is to use a narrow-necked jar and fit another, smaller jar into the opening to create an airlock. Or, you can weigh the flowers down with a small plate or jar lid or something.
Keep in a cool, dark place for as long as you like, but at least a few days. I typically hold mine for two weeks, although I used to do a month. The longer you steep the flowers, the darker the liqueur gets.
Strain twice. First through a fine-meshed strainer to remove the flowers and debris. Then strain it again through the same strainer, only with a piece of paper towel set inside it. This second straining removes very fine particulates, like the pollen. You can skip this second straining, but your liqueur will be cloudy.
For a quart's worth, add between 1/4 cup and 1/2 cup sugar, depending on how sweet you want it. Seal the jar again and shake well to combine. If you are using Everclear, a good way to get the sugar into the liqueur -- and dilute it enough to make it drinkable -- is to mix the sugar with an equal volume of water, heat it until the sugar is completely dissolved, cool it back to room temperature, and then add it to the liqueur.
Put the jar back in the pantry, and shake it from time to time until the sugar has dissolved. When it is, you are ready to drink it. It will last forever.