Make Eevee your Buddy Pokémon
Walk 10 km with Eevee as your Buddy
Earn 2 Eevee Candy while Eevee is still your Buddy
Evolve Eevee at night in the game
Make sure the evolution button shows Umbreon before evolving
If the name trick has not been used yet, rename Eevee to Tamao and evolve it once for a guaranteed Umbreon