To evolve your Charcadet into Ceruledge in Pokémon Scarlet and Violet, you'll need to trade it while holding a specific item depending on the version you're playing:
For Pokémon Violet:
Collect Sinistea Chips: You'll need to gather 10 Sinistea Chips. These can be found by defeating or catching Sinistea Pokémon throughout Paldea.
Trade for Malicious Armor: Travel to the town of Zapapico in the eastern half of Paldea, located west of the Lurking Steel Titan challenge area. Find the woman standing by the fountain in the center of town.
Trade Sinistea Chips for Malicious Armor: Talk to the woman and choose to trade. Offer her the 10 Sinistea Chips you collected. In exchange, she will give you the Malicious Armor.
Equip Malicious Armor on Charcadet: Open your inventory, go to the "Other Items" tab, and select the Malicious Armor. Choose "Use this item" and then select your Charcadet to equip it with the armor.
Evolve Charcadet: Once Charcadet is holding the Malicious Armor, simply initiate a battle with any wild Pokémon or trainer. During the battle, your Charcadet will automatically evolve into Ceruledge