Evolve Eevee into Leafeon using 25 Eevee Candy near a Mossy Lure Module
Make sure you are within range of the PokéStop with the active Mossy Lure when evolving
Use the nickname trick once per account by renaming Eevee to Linnea, then evolve it with 25 Eevee Candy
If the nickname trick has already been used, the Mossy Lure method is the standard way to get Leafeon