Sweetening the wine you've brewed once the fermentation has completed is called 'Back Sweetening'. You can use this technique to sweeten wines that are too dry in order to better suit your tastes, or to create something akin to a dessert wine.
Your wine should be sweetened before it is bottled and after it has been clarified and the stabiliser tablet added.
The most important thing to avoid is the sugar added at the end of fermentation being fermented by the yeast and causing your bottles to explode.
You have two options for sweetening your wine.
Using a wine sweetener (recommended)
We would recommend sweetening your wine with a non-fermentable wine sweetener, such as the ones in the links below. These will add sweetness to your wine but as the sugars they contain can't be digested by the yeast, there's no risk of restarting fermentation.
Using sugar or grape juice (not recommended)
An alternative method is to use sugar (dissolved in a 1:1 ratio with water) or grape juice. This will give you sweetness to your desired level, and also some added flavour if you use the grape juice option. However doing this does risk the fermentation restarting if the yeast wasn't completely deactivated by the prior fermentation.