Things You'll Need
Instructions
Obtain Eevee in either "Pokemon FireRed" or "Pokemon LeafGreen." You can get Eevee by going to the top of the second-tallest building in Celadon City, found near the Pokemon Center. There will be a Pokeball containing Eevee on a table.
Trade the Eevee from "Pokemon FireRed" over to "Pokemon Sapphire." Insert the FireRed cartridge into one Game Boy and the Sapphire cartridge into the other one. Then connect the two Game Boy systems using the Game Boy link cable, and commence trading.
Increase your Eevee's happiness by having it fight in battles, keeping it from fainting, feeding it berries, and using lots of items on it. However, do so only when there is sunlight out, as Eevees will evolve in to Umbreons during the nighttime. Do not use any evolution stones on Eevee, as some stones cause it to evolve into other Pokemon. Eevees will evolve into Espeons when they are the happiest in the daytime.