The basic idea is to start with the default outfits which you'll already have unlocked when you start playing. For regular costumes, some older characters have their classic dw7 looks ready to use, but others might need special codes or dlc especially for promotional stuff like that 7up zhao yun outfit. Most of the really cool stuff comes from buying dlc packs, like those original costume sets and if you're into the fantasy designs you'll definitely want the expansion content.

When you're running through story mode, keep in mind you won't be able to use alternate outfits there, but they work fine in other modes. Ambition mode lets you play dress-up all you want which is pretty nice for showing off your collection. Something interesting to note is that the complete edition comes with way more options if you're looking to expand your wardrobe.

Tip: if you're planning to collect everything, you might want to check the dlc listings first since quite a few outfits are locked behind those purchases and there's no way to unlock them through regular gameplay.