I learned this while working as a baker in a major whole foods grocery. It is the only brown sugar I use now.
I scoop about 5 or 6 cups of granulated sugar into a mixer bowl. Pour about 1/4 cup of good quality molasses over the sugar.
With mixer on low speed, stirring the molasses into the sugar takes about 5 minutes to incorporate it well enough. Be sure to scrape sides with a rubber spatula to get all the molasses into the sugar.
It should be a lovely color and very moist when done. I store in a glass jar and it keeps very well, though I use it up quickly. The brown sugar is exactly what you would buy in a box at the market, but with a more intense flavor and moistness.