In order to evolve Eevee into one of its Generation II evolutions you will need 25 candies and a specific nickname (without which its evolution will be random).
- Espeon: Give your Eevee the nickname Sakura
- Umbreon: Give your Eevee the nickname Tamao
If you’re more into the old-school Eeveelutions, here are their nicknames:
- Vaporeon: Give your Eevee the nickname Rainer
- Jolteon: Give your Eevee the nickname Sparky
- Flareon: Give your Eevee the nickname Pyro