Level up Eevee with at least 2 Fairy-type moves learned
In games that use affection/friendship, raise Eevee’s friendship to the required level
In games that require it, make sure Eevee knows a Fairy-type move before evolving
Level up Eevee after meeting the evolution requirements
In games where evolution is triggered from the party menu, select Eevee and evolve it when the Sylveon option appears