Evolve Eevee into Sylveon by raising Eevee’s affection/friendship high enough
Make sure Eevee knows at least one Fairy-type move
Level up Eevee after meeting both requirements
In some games, use a Soothe Bell, camp, picnics, or similar friendship-boosting methods
Avoid having Eevee learn only non-Fairy moves if you want Sylveon specifically
In Pokémon GO, use 70 Buddy hearts or the nickname trick if available
In Pokémon Scarlet and Violet, use a Fairy-type move and high friendship, then level up
In Pokémon Sword and Shield, use high friendship and a Fairy-type move, then level up
In Pokémon X and Y, use high affection and a Fairy-type move, then level up