Originally Posted by CWoodard
I'm not sure why everyone leaves this out
Add the sugar - preferably dissolved in the smallest amount of water already - after a day or two or three in primary. This will keep your yeast in a lot better condition. Make your hydrometer your best friend and add the sugar as soon as your gravity is down to 50% of final.
Adding sugar mid-ferment might be a good option in certain cases, such as avoiding excessively high OG's, but I doubt many cidermakers would consider it preferable in most ciders. Cider has a relatively low OG, so adding a moderate amount of sugar isn't too stressful for the yeast.