The recipe is simple: 1 part white granulated sugar, 4 parts water. That's it.
Don't buy pre-made nectar from the store. Commercial nectar often contains food coloring or preservatives that can be harmful to hummingbirds — and it costs far more than making your own.
I use C&H Pure Cane Sugar — nothing special about the brand, but it's a reliable refined white granulated sugar, which is exactly what you want. Avoid raw sugar, organic sugar, honey, or any artificial sweeteners. Raw and organic sugars contain extra minerals, including iron, that aren't good for hummingbirds. Honey can harbor bacteria. Artificial sweeteners provide no nutrition.
Never add red food coloring. The red on your feeder is enough to attract hummingbirds. There's no benefit to dye, and it may cause harm.
My Method
I boil filtered water in a kettle and pour two cups into a saucepan. Add one cup of sugar and stir until completely dissolved. Then add two more cups of cold filtered water and stir again. That makes one batch — enough to fill three feeders. If you don't have filtered water, boil all four cups first, dissolve the sugar, and let it cool before filling.
Store extra nectar in the refrigerator for up to two weeks. In hot weather, change and clean the feeders every two to three days — in cooler weather, once a week. Don't leave it longer than that. See Neglected Feeders Kill Hummingbirds for why that matters.
A slightly stronger mix (3 parts water to 1 part sugar) can be used in cold weather to lower the freezing point, or to help attract birds to a new feeder. Stick to the standard 4:1 ratio the rest of the time.