First, mix a thick syrup with your choice of granulated or brown sugar, molasses, honey, maple syrup, or a mixture thereof. I like to go 2 parts of sweetener, to 1 part of water.
Boil your syrup for a few minutes. You want your sweetener to dissolve, and you also want to sterilize it AND the water.
Have the amount of wine you’re looking to sweeten in a single, sanitized vessel - usually the carboy you stabilized it in.
Use a sanitized wine thief, turkey baster, or siphon to get a bit of the wine in a glass.
Taste the wine, decide if you’re looking at needing a lot of syrup, or just a bit.
Eyeball it and add some syrup to the carboy. You’ll want to underestimate, if anything. You can always add more syrup, but you can’t *un* sweeten a wine you went overboard with!
Swirl the jug to mix it in well.
Get a bit more wine in a glass, test it out. Add more sugar if needed, repeat as necessary - just be sure to keep everything sanitary!