In a medium sized bowl, combine granulated sugar and molasses. Add 1 tablespoon of molasses for light brown sugar or 2 tablespoons for dark brown sugar.
Mix together until the sugar is evenly coated in the molasses and it resembles store-bought brown sugar. It'll clump together at first, but just keep mixing and it'll come together.* Note: It'll be slightly lighter in color than store-bought brown sugar, but it still works in any recipe as an equal substitute.