Set Eevee as your buddy and earn 70 hearts
Make sure Eevee knows a Fairy-type move is not required
Collect 25 Eevee Candy
Evolve Eevee during the day or night
For the one-time name trick, rename Eevee to Kira before evolving
If you use the name trick, evolve it once after renaming to get Sylveon
If you do not use the name trick, the buddy-heart requirement must be completed before evolving