1. Catch an Eevee. Your first step is to find and capture an Eevee in the wild. Eevee can be found in the following locations in Pokemon Diamond:
- Route 206: Eevee can be encountered as a rare wild Pokemon.
- Trophy Garden: After obtaining the Trophy Case from the Sinnoh Museum, wild Eevees can be found in the Trophy Garden located east of Eterna City.
2. Increase Eevee's Friendship: To evolve Eevee into Espeon, you must raise its Friendship level. Friendship can be increased by doing various activities, such as:
- Battling with Eevee in your party
- Using Eevee to interact with Berries and other items
- Feeding Eevee Berries and Poffins
- Grooming Eevee
3. Level Up During Daytime: Once Eevee's Friendship is sufficiently high, you need to level it up during the daytime (in-game time). Make sure it's daytime in the game when you level up Eevee.
4. Espeon Evolution: When you level up Eevee during the daytime with high Friendship, it will evolve into Espeon.
Note that if Eevee levels up at night with high Friendship, it will evolve into Umbreon instead.
By following these steps, you can successfully evolve your Eevee into Espeon in Pokemon Diamond.