You should spend some time in the
Skeeter Pee forum, the
Country Fruit Winemaking forum, and/or the
DangerDave's Dragon Blood Wine thread. There is a wealth of information there about making interesting and often inexpensive wine, and how and why to backsweeten properly.
Very basically, you only start with enough sugar to get the alcohol level you want and that your yeast can handle. You ferment it until dry, which makes it very easy to tell when all of the yeast is dead and the sugar is completely gone. Then you stabilize it to prevent possible refermentation, and let it clear. Take the clear wine and add as much sugar and/or other flavours to your taste. Wait a while to make sure it doesn’t start fermenting again, and then bottle it.