Port and sherry are called “fortified wines”, because alcohol is added to a fermented wine to prevent it from turning to vinegar when exposed...

In order to produce wine from grape juice, you must allow the grape juice to ferment. Fermentation is the process by which yeast converts sugar into alcohol. Fermentation will continue to occur until one of two things happen: 1. the yeast runs out of sugar or 2. the alcohol concentration reaches a level which is toxic to the yeast and the yeast dies.

To produce a fortified wine, extra alcohol must be added. This alcohol...

In order to produce wine from grape juice, you must allow the grape juice to ferment. Fermentation is the process by which yeast converts sugar into alcohol. Fermentation will continue to occur until one of two things happen: 1. the yeast runs out of sugar or 2. the alcohol concentration reaches a level which is toxic to the yeast and the yeast dies.

To produce a fortified wine, extra alcohol must be added. This alcohol is normally obtained by distillation. 

So, if you are given as a starting point only grape juice containing water, sugar and yeast, you must first allow the mixture to ferment and create wine. Then take a portion of the wine and distill the alcohol from it. Then add this distilled alcohol into the remaining portion of wine.