Okay, give a pokemon a Rare Candy to hold. Save your game outside a PC in a Pokemon Centre. Now place it in a Box in a PC (and make sure that there's at least one space in your party). After you have done that, without turning off the PC at all, go to switch the pokemon holding the Rare Candy to another Box. You have to save to do this, so save, but when the message 'Saving...Don't Turn Off The Power'. However, as soon as 'Saving...' pops up, turn off the power. Turn your game back on, load it up to your most recent save (just outside the PC). The pokemon holding the Rare Candy should still be in your party. However, check the PC and you'll find that there's a duplicate copy of the pokemon holding the Rare Candy (which was cloned with it), so you can remove the clone pokemon from the box and take the Rare Candy from it. Repeat the process for more Rare Candies.
That should sum everything up. While I'm at it, I'll move this topic to the correct forum. Since you appear to be new to the forums, I recommend that you read the Forum Rules. Moving...
Sig and avatar by Ozzo. Thanks.