1. Catch or encounter all of the Unova Pokémon:
You must catch or encounter all 156 Unova Pokémon (from National Pokédex #494 to #649) in Pokémon X. This can be done by catching these Pokémon in the wild, trading with friends, or transferring them from other Pokémon games.
2. Defeat the Pokémon League Champion:
Once you have completed the Unova Pokédex by catching or encountering all of the Unova Pokémon, you will need to defeat the Pokémon League Champion, Diantha, in the Pokémon League.
3. Receive the Cute Charm from Professor Sycamore:
After you defeat Diantha and become the Champion, return to Professor Sycamore's laboratory in Lumiose City. He will recognize your accomplishment and reward you with the Cute Charm.
4. Equip the Cute Charm:
To take advantage of the Cute Charm's effect, you will need to equip it on one of your Pokémon by selecting it from your Bag and using the "Hold Item" option.
The Cute Charm, when held by a Pokémon, increases the chance for wild Pokémon encounters that can result in hatching a Shiny Pokémon. It's a valuable item for shiny hunting and completing your Pokédex.