The journey of Elaina - Official Terraria Mods Wiki
To access the content granted by the mod you have to name your character Elaina (with uppercase E, otherwise it won't work), you will know it works just by looking at your character's vanity slots, both in the menu and in the gameplay.
Is there a way to have more accessory slots? : r/Terraria
Besides, the vanity accessory slots allow for extra accesoriies to be swapped in and out when needed.
Vanity items - Official Calamity Mod Wiki
Personally I think that a separate mod enabling only the set bonuses in your vanity slots could be cool. Just so you could mix and match your armor and get a set bonus.