Here's my method. I don't remember what was the source, but it works well:
First, be very sure the wine is stabilized before adding sugar to it or it will start fermenting again. One crushed and dissolved Campden tablet and 1/2 tsp of K-sorbate (also dissolved) per gallon of wine will do the trick.
Second, you can sweeten with just sugar or you can make a simple syrup with two parts sugar dissolved in one part of water (as in two cups of sugar in one cup of water). Boil the water, remove from the heat, add the sugar, and stir like heck to make the syrup, as that much sugar doesn't easily dissolve in cold or warm water. Do NOT use commercial powdered sugar, as it contains corn starch which can permanently cloud your wine. Allow the simple syrup to cool to room temperature (not in a refrigerator or it might start re-crystallizing) before continuing.
Third (this might sound complicated when you first read it, but it's really not), measure how much liquid it takes to fill your hydrometer test jar to within 3" of the top. It take about ¾-cup to fill mine that far. Measure out that much wine into a large water glass and stir into it 2 tablespoons of simple syrup. Fill the hydrometer test jar with this sweetened wine and measure the SG. Write that number on a piece of paper and set a wine glass on top of the number. Pour about one inch of wine from the hydrometer test jar into that wine glass and pour the remaining wine back into the large water glass.
Replace the amount of wine you poured into the wine glass so you have as much as you started with and stir into it 2 more tablespoons of simple syrup. Again pour it into the hydrometer test jar and measure the SG. Write the number on a piece of paper and again set an empty wine glass on the number. Pour an inch of wine into the glass and return the rest to the water glass.
Again replace what you used and add 2 more tablespoons of simple syrup. Stir, pour into the hydrometer test jar, and repeat the previous procedures. Do this until you have 4-5 wine glasses sitting on their SG figures. Now taste them in the order they were filled (first glass to the last) and note the one that tasted best to you. It will be the one you tasted just before you picked up the one that was too sweet. Look at it's SG -- that's the SG you want to sweeten your wine to.
Here's another consideration. Over time, all wines mellow out somewhat and actually taste a little sweeter that they did when first bottled. If you plan on keeping the wine for a couple of years, you might want to back off the target sweetness just a hair to allow for this. For example, if the target SG is 1.012, you might want to sweeten it to 1.011 or even 1.010 to allow for this perception.