Brown sugar is simply granulated white sugar with molasses added to it. To mix your own, simply place the granulated sugar in a bowl and with your mixer running, gradually add the molasses, mixing until it’s evenly distributed. You can make brown sugar by hand by simply stirring the molasses into the sugar, but the mixer produces a smoother, more even product.
The result should resemble the brown sugar you buy. Adjust the amount of molasses to suit your taste, using less for light brown sugar and more for dark brown.
Your homemade brown sugar will have a slightly more pronounced molasses flavor than the store bought version, but in most cases the additional flavor will be a good thing.