Combine the sugar and molasses in a bowl and use a fork to mix. You can also mix them in a food processor or in a plastic bag. Add more molasses as needed to obtain the desired color and flavor. 1 tablespoon of molasses will give you a light brown sugar while 2-3 will make a dark brown.
If you don't have molasses you can use maple syrup, golden syrup or agave syrup. All of these will ad a similar texture/amount of moisture but they will have different tastes.Store the brown sugar in an airtight container.Don't be afraid to use your hands to mix the molasses in, it's the most efficient way if you're not using a food processor.