There are three methods available to you. The first involves putting your juice through secondary fermentation.
Mix some priming sugar into your hard cider before bottling it to begin this fermentation. This won't require any extra prep; Just add the sweetener of your choice.
You'll need roughly 2.5 tbsp of sweetener per gallon of apple cider you're carbonating.
Ensure that your cider is inside bottles that can take extra pressure without popping. We recommend using flip-top bottles for this method!
After you've bottled your drinks, wait about 2 weeks for them to carbonate. Expect to see some extra sediment in your bottles. And they're ready!
The second and third methods are much easier. Just pick up some carbonation drops, and make sure they'll fit the size of your bottles. Follow their directions to get it right!
Finally, you could use force and some more tools. Pick up a Corny keg, and pump carbon dioxide directly into your juice.
This method also allows you to back sweeten if you wish.