Ensure you have a Sylveon candidate: use an Eevee with at least 70 hearts with you
Earn hearts by walking with Eevee, feeding it berries, and playing with it at Buddy level
Check that the Eevee has the Fairy-type move “Charm” (or evolves after it learns “Charm”)
Give the Eevee a nickname that triggers Sylveon evolution: “Kira” (one-time name trick)
Evolve the Eevee while it meets the requirements to get Sylveon
If the nickname method doesn’t work, use the 70-heart requirement plus having “Charm” and then evolve normally