It's awesome and useful!
Anyway, you take your Pokemon and go to the Battle Tower in the Frontier. You open your PC in the Tower and deposit all the Pokemon you want to clone in a preferably empty box. Then you exit the PC and save. Then take the Pokemon back out and go talk to the lady that does double link battles, I think it was. She is the one on the far right of the Tower, closest to the PC. You talk, accept, pick your Pokemon and when it asks do you want to save you say yes.
Now if there is an odd 2 second pause before the actual saving screen pops up, it means that so far it's all good. When the screen actually pops up, after the two seconds, turn the game off. When you turn it back on you will have the Pokemon with you as well as those in the PC. I do it very often and it never corrupted a save or anything. Items are cloned this way as well by the way.
I could'ave typed some stuff incomprehensibly, so just try with say a Zigzagoon first before cloning major stuff.