How to Get Every Eevee Evolution in Pokémon GO
Eevee, the evolution Pokémon, has the mysterious ability to evolve into one of several different Pokémon. If you're wondering how to evolve Eevee into each one of its Eeveelutions in Pokémon GO, you've come to the right place! This guide will cover each method available for evolving Eevee into the Eeveelution you desire.
The Nickname Trick
By far, the easiest method to evolve Eevee into the Eeveelution you want is to use the nickname trick. By nicknaming Eevee one of the names below, you can guarantee it will evolve into the corresponding Eeveelution. However, this method can only be used once for each Eeveelution, so it is highly recommended you choose your Eevee wisely when opting for this method.
To evolve, tap the pencil icon next to your Eevee's name to change its nickname. Once you input the name and tap OK, the question mark on the Evolve button should be replaced by an image (or silhouette if you don't yet have the Pokédex entry) of your target Eeveelution. If not, you either typed the name in wrong, or you have already used the nickname trick for that Eeveelution.
Vaporeon -> Rainer
Jolteon -> Sparky
Flareon -> Pyro
Espeon -> Sakura
Umbreon -> Tamao
Leafeon -> Linnea
Glaceon -> Rea
Sylveon -> Kira
Random Chance
By simply tapping the Evolve button and spending 25 Eevee Candy, you have a 1/3 chance to evolve Eevee into either Vaporeon, Jolteon, or Flareon. Unless you've met certain requirements, Eevee will only evolve into one of these three Pokémon using this method.
Conditional Evolutions
If you've met the required conditions, your Eevee may evolve into one of its other Eeveelutions. You'll know you've met the conditions if the question mark on the Evolve button is replaced by an image (or silhouette if you haven't registered the Pokémon in your Pokédex) of that Eeveelution. Each evolution will require you to have at least 25 Eevee Candy in your inventory.
Espeon and Umbreon
To get Espeon or Umbreon, you must have Eevee set as your buddy, and you must have walked at least 10 km with that Eevee and earned 2 Eevee Candies. Once this condition is met, you can tap the Evolve button to evolve Eevee into Espeon during the day, or Umbreon during the night.
Leafeon and Glaceon
To get Leafeon or Glaceon, simply evolve your Eevee while in proximity of a PokéStop with an active 
Mossy Lure Module or 
Glacial Lure Module, respectively.
Sylveon
To get Sylveon, you'll need to set your Eevee as your buddy Pokémon. Once you've earned 70 Hearts with Eevee as your buddy, it can evolve into Sylveon.