This article is from the WineMaking FAQ, by malak@CAM.ORG (Don Buchan) with numerous contributions by others.
There are many ways of sweetening your wine.
A) When your wine has reached about 1.000, put in your sulphite,
sorbate and clearing agent. This will usually give you only a slightly
B) Ferment your wine dry, then stabilize it with your sulphite and
sorbate, and add your clearing agent. Filter the wine. Then add a 2:1
sugar/water (or wine) syrup that contains sulphite and sorbate.
C) Add some glycerine. This technically won't sweeten the wine but it
will add to your perception of sweetness.
D) Blend the stabilized wine with a stable over sweet wine.
E) Add sugar to the wine just before serving. Two teaspoons per bottle
will increase the sugar content by 1%, and 4% will approximate port.
As far as what degree of sweetness you want, add a little bit at a time,
and stop when it tastes like it could use just a bit more.
< 0.5%; <1.000 SG; 1 = dry < 3%; <1.010 SG; 2 = medium dry < 5.5%; <1.020 SG; 3 = medium sweet < 8%; <1.030 SG; 4 = sweet <10.5%; <1.040 SG; 5 = dessert