Things You'll Need
Instructions
Get an Evee. If you're playing "Pokemon Fire Red" or "Pokemon Leaf Green," Evee is easy to find. With an empty slot in your party, go to Celadon City. At the north end of the city is a path leading behind the buildings. Follow this path and you can go into one of the buildings through a rear entrance. Go to the top floor; here you'll find Evee.
Take your Evee to the second floor of any Pokemon Center.
Connect your Game Boy Advance to another Game Boy Advance containing "Pokemon Ruby," "Sapphire," or "Emerald." The game-link cable plugs directly into the side-port located on your Game Boy Advance.
Using both games, talk to the Pokemon Center attendant with the image of two Game Boy Advance systems next to each other and trade Evee to Pokemon Ruby, Sapphire, or Emerald.
Raise Evee's happiness as much as you can. Do this by walking around with it in your party, never putting it in a PC box, and giving it vitamins like HP Up, Carbos, Calcium, Iron, Protein and Zinc. Use it in gym leader battles or against the Elite Four.
Evolve Evee during the day. If you level it up at night accidentally, it turns into Umbreon. Cancel the evolution by pressing the "B" button and wait until the afternoon to level it up again to evolve it.
Trade Espeon back to "Pokemon Fire Red" the same way you traded it to "Ruby," "Sapphire," or "Emerald."