First you will need some potassium sorbate (0.5 tsp / gal) to add to the wine to assure the wine doesn't start fermenting again when you add the sugar. Some sulfite's (Campden, K-Meta, Na-Meta) to prevent oxidation. Use a test kit to check the pH of the wine and add sulfite's to get about 20 ppm free sulfite's.
Then make a simple sugar solution. I don't know how big a batch you have so it will depend on how much solution to make. For example, for a gallon I would take a couple cups of sugar and add to 0.75 cups of boiling water and stir until totally dissolved. Add some, stir and taste. Keep adding a little at a time until desired sweetness is obtained. Remember though, a little at a time. It can catch up on you pretty quick. If sulfite levels are good, you significantly reduce the chance of oxidizing the wine.