To unlock the High Fashion trophy in Borderlands: The Pre-Sequel, you will need to unlock 10 customisation items.
Customisation items are most commonly found by looting dead enemies/creatures. You can also win them by using the slot machines found in Moxxie's Up Over bar in Concordia.
Once you have a customisation item, go to your inventory and press
to unlock it. Once you have done this 10 times, the trophy will unlock.