On top of a cutting board, slice the skin off one of the lemons with a vegetable peeler or paring knife, being sure to only get the yellow part, not the pith. The pith is the white part under the peel and can make the syrup bitter. Set the skin aside in a small bowl or container.

Juice the lemons into the measuring cup, until you have half a cup of juice. This could take 2 or 3 lemons, depending on how juicy or large your lemons are.

Put the lemon juice into a medium saucepan and place it over medium-high heat. Pour the sugar into the juice and stir with a whisk or wooden spoon until fully combined. Bring the mixture to a boil, then reduce heat to low and let the mixture simmer until slightly thickened. This will take at least 10 minutes, possibly up to 15. Be sure to stir the mixture occasionally so it thickens evenly.

After the mixture has thickened, stir in the lemon zest then transfer to a bowl to let cool for roughly 20 minutes.

Once cooled, remove the skins with a pair of tongs, or if necessary place a fine mesh strainer over a mason jar (or other container) and pour the syrup in. Place the lid on the jar and store the syrup in the fridge. The syrup can keep in the fridge for at least one month.