Catch an Eevee
Walk Eevee as your buddy for 10 km
Earn 2 Eevee candy while Eevee is your buddy
Keep Eevee as your buddy
Evolve Eevee during the day in Pokémon GO
Make sure Eevee is still your buddy when you evolve it
Use the nickname trick once by renaming Eevee to Sakura before evolving it to guarantee Espeon