Image Source: Everett Collection
According to Reddit user smithnigel, you can make your Pokémon evolve into one of the three forms you want it to become: Jolteon, Vaporeon, or Flareon. All you have to do is nickname your Eevee the corresponding Eevee Brother it's associated with. In the show, the Eevee Brothers include Sparky (Jolteon), Rainer (Vaporeon), and Pyro (Flareon). After renaming your Pokémon to reflect your desired evolution, smithnigel advises that you restart the app to be safe. Based on the positive feedback users have been providing, this tip seems to be 100 percent effective.
Go on and try it yourself!