#1: Make sure you have 2 3DSs, it doesn't matter if they're both yours or if you're doing this with someone else. Make sure one has the Pokemon you want to duplicate while the other has a Pokemon they won't mind getting rid of. Also make sure you're friends or can trade in any way besides the Wonder Trade or GTS.
#2: The 3DS with the crappy Pokemon starts the trade. You trade the crappy Pokemon with the one you're duplicating. Proceed trade normally until you both have the Pokemon.
#3: The screen now changes to a screen that says "Connecting, please wait...". The 3DS who now has the crappy Pokemon waits approximately 4 seconds, then turns off their 3DS. After a bit, on the other screen it will say there has been connecting problems. Make sure the screen is black and does not have an OK option, or else it for sure did not work, but even if it does have a black screen, it still may not have worked.
#4: Repeat process until Pokemon is duplicate. Note the Pokemon that is not the one you are duplicating will be gone, so make sure it's crappy or something. Also, remember that this is a long, hard process. But also remember that when it does work, you will be very happy!

So if you like this please leave a comment and tell me which awesome Pokemon you have been duplicating! Have fun doubling awesome Pokemon!

P.S. So far I have duplicated a Moltres and a Mesprit. Yes, a Mesprit. And awesomely French one.