Instructions
Trade an Eevee over from Pokemon "Fire Red" or "Leaf Green." Place this Pokemon into your party.
Use Eevee in battles, or simply keep it in your party to up its Happiness rating. Use items to up the rating as well.
Travel to Verdanturf Town and speak to the Friendship Rater about Eevee. When the Rater tells you that Eevee's happiness is at its maximum level, save and go train the Pokemon.
Level Eevee between the hours of noon and midnight in order to trigger the evolution to Espeon.