Add a few cups of wine to a pot, add 3-4 ounces of sugar. Disolve then use that to determine the correct sweetness for your taste and take a hydrometer reading. You can mix the dry wine and sweetened wine in a beaker.
Once you have found the correct specific gravity, add all of the sugar wine to your carboy and take a reading. Calculate how much sugar you need then make up in the pot what you need.
You will notice once the sugar is fully dissolved there will be white streaks in the wine. That is sugar not fully dissolved. Continue stirring until all is dissolved. Wait about 30 seconds more and turn off the heat. Heat on the lowest flame or setting. Never even bring it to simmer, just warmed up good.
Leave it in the pot a few minutes more then blend back into your carboy. Take a final gravity reading and record for your records.
I like to aim low on the amount of sugar needed as I can always add extra. 4 ounces of sugar by weight will raise one gallon of wine .010
If your wine is less than .990 you will need to get a reading up to .990 or above to be accurate.
Allow your wine to rest a few weeks and if no refermentation you may bottle. Recommend adding some pot. sulfite, and pot. sorbate to prevent yeasts, if any from reproducing.