This is actually quite easy.
First let your ferment complete normally. You will have a dry wine.
Then add potassium sulfite (usually sold as Campden tablets) and potassium sorbate. These two chemicals used in combination will prevent renewed fermentation. Check with your kit it is possible that the kit includes these additives, they will be in the stuff you add after fermentation.
Then dissolve some sugar in boiling water (to pasteurize the liquid) and add it to the wine to taste.
After the wine clears bottle as normal. If you by a sweet wine kit this exactly what the F-Packs do.
A good way to determine how much sugar to add to the wine is to measure out several samples of the wine into separate glasses. Add a different amount of sugar to each one and then taste. When you find your favorite ratio, scale up the addition to the batch size and add the appropriate amount. It is best to aim a little dry on your addition as you can always add more sugar but you can't take it out. Yooperbrew claims that after aging in the bottle the wine will seem a little sweeter than it did initially.
Craig