It's really easy. You need an account on the Pokemon Global Link first.
Here I'm gonna use a Keldeo C-Gear Skin as an example:
1) Log in to the Pokémon Global Link, click on "Promotions"
2) Click on "Get the Keldeo Ordinary Form C-Gear Skin!"
3) Enter the password KELDEOCGEARBW to obtain your new skin.
Same with Pokemon Event's too, do the same procedure.