You will need to add sulfite and sorbate to your wine to stabilize it before adding the sugar or the yeasts will eat the sugar and you will have a stronger dry wine.
sulfite is Potassium metabisulfite usually sold as Campden tablets
sorbate is potassium sorbate
If you have not added any sulfite to the wine add 1 Campden tablet per gallon to the wine (crushed and disolved) and 1/4 teaspoon sorbate per gallon. Then add your sugar to taste. I have seen several different schedules for adding the additions from adding all at the same time to adding them over several days. Since the instructions with my wine kit added everything at the same time that is what I have done on subsequent homemade batches.
<edit> looks like Yooper beat me to the draw with better instructions and more experience.