Level up Eevee with high friendship during the night
In Pokémon Sword and Shield, use a Soothe Bell and keep Eevee in your party
In Pokémon Scarlet and Violet, increase Eevee’s friendship and level it up at night
In Pokémon Legends: Arceus, use a Friendship evolution method at night
Do not let Eevee know a Fairy-type move if you want Umbreon instead of Sylveon
Make sure Eevee is not holding an Everstone
Check that Eevee’s friendship is high enough before leveling up
Level up Eevee after reaching the required friendship and nighttime condition