Catch an Eevee
Set Eevee as your Buddy
Walk 10 km with Eevee as your Buddy
Earn 2 Eevee Candy while Eevee is your Buddy
Evolve Eevee at night in the game
Make sure Eevee is still your Buddy when you evolve it
Use the name trick once by renaming Eevee to Tamao before evolving it, if available