1 cup of regular sugar
1 tablespoon of molasses for light brown sugar or 1.5-2 tablespoons for dark brown sugar
Take the two ingredients and mix them.
You can use an electric mixer, a bowl and fork, spoon, your hands (I don't really recommend this), etc.
I prefer to use my mixer and do about 3 cups at a time.
Store it in an air tight container. This is a nice way to have soft fresh brown sugar that you don't have to take a hammer to in order to use.
***Tip: if you do have some rock hard