1st Generation Eeveelution Methods
Unlike every other Pokemon in Pokemon GO, the method to obtaining a certain eevee evolution is at complete random. If a player was to evolve an Eevee without doing anything to that Eevee, the player would get either vaporeon, jolteon or flareon (not espeon or umbreon). However, there is a way for players to have one guarantee to evolving into any of the 1st Generation eeveelutions, and it rests solely upon nicknaming the Eevee a certain name (first discovered by a fellow gamer back in the 1st Generation of Pokemon GO).
If a player were to nickname their eevee with the name of Rainer, it will evolve in vaporeon; if a player were to nickname their Eevee with the name of Sparky, it will evolve in jolteon; and if said player were to nickname their Eevee with the name of Pyro, it will evolve in flareon. But do remember that this trick only works for each eeveelution once; after that, even if you nickname it one of the names mentioned here, the Eeveelution you receive will be at random.