Make a simple syrup of 1 cup water to 2 cups sugar and fully dissolve. Make sure you sulfite and sorbate before adding to your pee. I'm not sure about timing but I did it on all my batches right before bottling.
I like to take a bottle of wine and take a half a cup at a time and add simple syrup in spoon measures. Make a bunch of test batches in wine glasses and get somebody to help you taste. Figure out which ones you like then make one somewhere in between the measures and pick the best of the three. I found that I preferred my last batch at 1/2 teaspoon syrup to 3/4 cup of wine. I was able to convert these numbers and add the right amount of syrup to the entire batch after the bench test. Even with those numbers, add a little at a time and taste often. Use onlineconversion.com to convert the teaspoon to cup measure.
Hope this helps!