1. **Raise Eevee's Friendship:**
- In the main series Pokémon games, Sylveon evolves from Eevee when it has a high level of friendship (also known as happiness). You can increase Eevee's friendship by:
- Leveling it up.
- Walking with it in your party.
- Giving it massages in certain Pokémon Centers.
- Feeding it vitamins, berries, or other items that boost friendship.
2. **Teach Eevee a Fairy-Type Move:**
- Eevee needs to know a Fairy-type move to evolve into Sylveon. In the main series games, Eevee can learn Fairy-type moves like Baby-Doll Eyes, Charm, and others by leveling up or using TMs (Technical Machines).
3. **Level Up Eevee:**
- Once Eevee has a high level of friendship and knows a Fairy-type move, level it up. Upon leveling up, Eevee should evolve into Sylveon.
4. **Other Requirements:**
- In some Pokémon games, additional conditions may be necessary for Eevee to evolve into Sylveon. For example, in Pokémon Sword and Shield, Eevee also needs to be near a Pokémon Camp when leveling up with a high friendship and knowing a Fairy-type move.
Keep in mind that the specific requirements for evolving Eevee into Sylveon may vary slightly depending on the Pokémon game you are playing. Be sure to check the game's specific evolution mechanics and conditions for evolving Eevee into Sylveon.