Recipe makes 1 cup (16 tablespoons) of syrup. Use 1- 2 tablespoons of syrup per 1 cup of coffee.
It's not necessary to boil the water. You can even stir brown sugar in hot water until the sugar dissolves.
Light or dark brown sugar can be used. The difference between light and dark brown sugar is how much molasses is in it and the more there is, the more caramel and toffee flavor the syrup will have. If you want a richer, more caramel flavor, go for dark brown sugar since it contains more molasses.
If you want to give your brown sugar syrup an upgrade, add a little vanilla extract, which I consider a flavor enhancer. It's not necessary, but if you have it, use it. Add a teaspoon of vanilla extract to the recipe.