Yeasts eat the sugar in the must and multiply. Then they convert the sugar they ate into alcohol and carbon dioxide (CO2). Under normal circumstances the conversion is 53% alcohol and 47% CO2, along with some other trace compounds. Now, the fun begins. When the alcohol level gets too high, it kills the yeasts! Depending on the yeast strain, this occurs anywhere from 12% - 20%. As a result, the trick is to make sure you start with a high enough sugar level to make sure you generate enough alcohol before it kills the yeasts. Most books, I have read, usually suggest a starting S.G. (specific gravity) of 1.080-1.090.

Now, you are probably saying that you can add sulfite and sorbate to stop the fermentation process and if you have been led to believe this will work, I say, you have been mislead! It is extremely difficult to stop an active fermentation. Sulfites and sorbate are used to prevent fermentation from starting again, not stopping an active fermentation. You can try to add sulfite to stop the fermentation, but in the process, you might have to add so much sulfite as to ruin your wine. The other way to stop an active fermentation is by "cold stabilization". I won't go into the details, but the general idea is to almost freeze your wine for several days to stop the fermentation.

All this being said, if you want a sweeter wine, be it from a kit or from fresh ingredients, here is my recommendation.

I will add that there are other ways to achieve the same result. I just like this method best as it leaves the least amount of room for error and virtually guarantees your wine will be exactly as sweet as you desire. In addition, if you do not want to mess with creating a sugar solution, I have a wine conditioner that can be used to accomplish the same result.