You can't do it until you've beaten the Champion. You'll then receive an item called the Vs. Recorder when you arrive in Kiloude City, which you can only reach in the post-game. You access it from the Key Items pocket of your Bag. You select it, choose Use, then hit the code button to input the code. Likewise, whenever you complete a battle online, you'll be asked whether you want to save your video. You can save up to 100, and you can upload 12 to share with others at a time. Uploading them provides a code.
it was really close, both of us in the red and on our last pokemon. That's the kind of battle I like.
