I start by combining the sugar and water in a small saucepan and setting it over medium heat. I whisk it constantly so the sugar doesn't clump or scorch on the bottom of the pan. Once the mixture comes to a gentle simmer and the sugar has fully dissolved, I turn off the heat right away.
Next, I drop the fresh basil leaves straight into the warm syrup. I use a fork to gently press them down until they're fully submerged, since any leaves poking out of the liquid won't infuse properly. This is the step that gives the syrup its signature herbal flavor.
I let the basil sit in the syrup for about 30 minutes off the heat. This resting time is what pulls all that fresh basil flavor into the liquid, so I try not to rush it even when I'm eager to use the syrup right away.
Once the steeping time is up, I pour the syrup through a fine-mesh sieve into a clean container, pressing the basil leaves against the strainer with the back of a spoon to squeeze out every last bit of flavor before discarding the leaves.
Finally, I let the syrup cool completely at room temperature before transferring it to an airtight container. Once it's cool, it's ready to use in any drink I like, or it goes straight into the fridge for later.