You just need to be famous. Really famous. Like this lady, Yuko Oshima. Not only is pop idol Yuko Oshima testing Final Fantasy XIII-2, she's also designing costumes. One of these outfits will be selected by fans as DLC for the game.
One of them looks like Little Red Riding Hood. The other does not.
Oshima is holding the final versions -- the ones that Square Enix prettied up. One of her original drawings can be seen in the gallery above.
As far as putting together an outfit for FF's Serah, it's fine, I guess. And she's got an eye for fashion. Remember, she's just supposed to be a tester, not a designer. The most famous Final Fantasy tester. Ever.
Image: Square Enix