Just as a more practical option for beginners (not that the above options are not good ones, they are superior to mine, but I am a lazy bastard and probably insane) add 1 lb of coconut sugar during the last 15 minutes of your boil. Coconut sugar ferments flawlessly and has lots of the nutrients found in yeast nutrient supplements. Also, the 1b for a 5 gallon batch will up your abv a bit more and not impart off flavors of any kind to my knowledge.
I have brewed over 200 beers and with many beers I want in the higher ABV range (7% plus) I use a pound of coconut sugar to make the yeast fondle each other like Jackrabbits. It's cheap, it's more sugar, and best yet I really don't use yeast nutrient add-ons anymore. In more traditional beers I use about two cups of it. For beefy Belgians and high ABV stouts and IPAs/DIPAs I go a single full pound.
I will say I never tried it during fermentation. However, some people say you can put sugar in there no problem. Hope this helps! b^^