Back sweetening wine is pretty simple. First stabilize the wine with some potassium metibisulfate if you haven't. Then add some potassium sorbate (follow directions on the package for how much to add per gallon). The sorbate will prevent yeast from reproducing and eating all the sugar you are going to add. Only having a gallon of wine to work with is going to be tough. Normally I would say pour several glasses worth and add different amounts of sugar to determine what you like. For one gallon, I would say make up some simple sugar, and sweeten in stages, tasting as you go to see what you like. Also fruit wines generally get more flavor as they age. I would also recommend stabilizing the wine and letting it set for another 6 months before sweetening if it's really young.