Instructions
Go to the Goldenrod Underground and save the game using the PC in front of the desk.
Choose Eevee as the first Pokemon in your party. Speak to one of the brothers behind the desk to give Eevee a haircut. This raises Eevee's happiness by a random amount.
Restart the game if you get the message "Eevee looks happy" or "Eevee looks a little happier." You are aiming for the message "Eevee looks delighted!". You can only get a haircut once a day, so you want to get this message each time. It means that Eevee's happiness went up by five to 10 points.
Give Eevee 22 EV-lowering berries. These berries are found growing around the region. When you take one, it grows back after a few days.
Raise your Eevee's level to at least 20 and let it hold a Smoke Ball. Head to Blackthorn City and walk around in the grass. Save the game. If you enter a battle with Geodude or Graveler, use the Sand Attack move until the Pokemon self-destructs. If the attack misses, Eevee gains a large amount of experience and happiness. If the attack hits Eevee, restart the game.
Play the game between 8 p.m. and 4 a.m. If Eevee is happy enough, it evolves into Umbreon.