Summarize this article in one sentence.
While Emerald is sorely lacking in the Eevee department, FireRed and LeafGreen both have a way you can get the Pokemon. If you want to get Eevee without resorting to cheating, you'll need to start a game of either and get at least as far as Celadon City. It's important to keep in mind you'll need to have collected at least 60 Pokemon and completed the Sevii Islands sidequest to open up trading. Once you've opened up the possibility for the Trade Centre, it's time to find Eevee. Eevee may be found in the Pokemon Mansion in Celadon City. Go to Celadon City, and enter the mansion via the backdoor. Get to the roof; on the roof, you'll find a room. Inside, you'll find Eevee's Pokeball. In order to trade, you'll need too Game Boy Advances, a copy of Emerald and FireRed or LeafGreen, and a game link cable to join the two. Get it all plugged in before you try to access the Trade Center. Ask a friend to borrow his Game Boy Advance; this will save you having to purchase another console in order to make the trade. With the two devices hooked up, you can now head to Vermillion City and access the Trading Center. Go up to the desk and speak to the hostess. From there, you can access the trade panel. Ensure the game link cable is joining the two consoles safely, then proceed to make the trade. The data will be erased on the LeafGreen/FireRed game, and copied onto Emerald. Now that you've made the trade, it's always good form to look on your Emerald file and make sure the file has gone through. If the trade has been made, you should see a little Eevee smiling back at you! If for some reason the Eevee isn't there, it's quite possible the trade wasn't completed. It's highly unlikely that one game would erase it without copying on the other, so try again and you should be able to get it down right.
Start a game of Pokemon FireRed or LeafGreen. Find the Eevee Pokeball in the Pokemon Mansion. Link up two Game Boy Advances. Go to the Vermillion City Trading Center. Make the trade. Verify on Emerald that the trade has gone through.