The first step is making sure the wine has stabilized and will not re-ferment in the bottle. Midwest also suggests adding one crushed Campden tablet per gallon to the wine and let it sit 24 hours before sweetening or bottling--to keep the wine from absorbing oxygen during the process.
Next is choosing the method. You can either (1) add sugar, (2) blend with a too-sweet wine, (3) use grape concentrate, or (4) blend with a sweeter juice. The choice is yours.
If you add sugar (by far the easier method), boil a measured amount of water and slowly dissolve a double-measured amount of sugar into it. The 2 to 1 by volume ratio is still the best. Sweetening a too-dry wine does not give immediate feedback. It takes a couple of hours to a day for the wine to fully absorb the sugar and integrate it into its character.
Add various amounts of sugar-water to your sterilized wine bottles and mark them with a Post-It note. For example, you might put 1-1/2 tsp in one, 2 tsp in another, 2-1/2 tsp in a third, 3 tsp in a fourth, etc. Fill the bottles and seal them with a tasting cork (t-cork).
Allow the bottles to sit about one hour and then measure the specific gravity of a sample from each bottle. Write this on the Post-It notes.
Now taste each sample. If they still retain the harshness of the dry wine despite having been sweetened, let them sit overnight and taste them the next day.
Decide which one you like best and add that amount of sugar-water to each bottle before filling it. Be sure to replace the t-corks with regular corks.
If you sweeten a too-dry wine, especially one with lots of tannin, try putting aside a few bottles of the dry wine to taste in a couple of years. They do mellow out, and once the sharpness mellows they are quite often excellent wines. If not, you can blend them in a decanter with a too-sweet wine when you drink it.