To name an Eevee for Sylveon, you can rename it to "Kira". This will guarantee that your Eevee will evolve into Sylveon when you have at least 25 Eevee Candy and the silhouette for Sylveon is on the evolve button. Its important to note that the naming trick only works once per evolution type, so make sure you choose an Eevee with the best CP values before evolving it into Sylveon.