First make sure the wine is stabilized, either chemically (by adding
potassiums metabisulfite and sorbate) or by sterile filtration. My
preferred way to sweeten to taste is to siphon off a sample and slowly
add sugar as simple syrup, stir, allow it a few minutes to assimilate,
and then taste it. If not right, add more and repeat. Simple syrup
is two parts sugar to one part water, dissolved to water's clarity.

I like to use a 380-mL sample size for a gallon-sized batch. This is
10% of the overall volume and whatever amount I add to it can be
easily calculated for the remainder (multiply it by 9 for the
remaining 90%). For a 5-gallon batch, I use a one-liter sample and
multiply by 18 for the remainder.

After sweetening the whole, take a hydrometer reading (make a note of
the reading) and to keep the wine under airlock for at least two weeks
and preferably three to ensure it does not start refermenting.

If you stabilized with chemicals and it shows signs of refermentation,
just wait it out. The yeast in the wine will not reproduce, but the
ones still in there will continue fermentation until they die out.
When the wine goes still, wait for the final dusting of dead yeast
cells to form on the bottom. Then add enough simple syrup to restore
the wine to the post-sweetening hydrometer reading, stir, let it sit a
few days for the dead yeast to drop out again, and carefully rack the
wine into bottles.

I like to mark the cork of the last bottle with an "L" so I know it is
the last one. This is the one that may have picked up a few of the
dead cells and throw a light dust in the bottle. I just like to know
which one it is so I don't give it to someone or bring it to a
competition.

Jack Keller, The Winemaking Home Page
http://winemaking.jackkeller.net/