How do you evolve Eevee into Leafeon?

Answer 1

You need to go to Eterna Forrest and go to the NorthWest of it.There is this moss covered rock, level up eevee near it by one level.Another way is to get a leaf stone from underground and give it to eevee.

Actually, you should go to the moss stone the holding the leaf stone will take forever 'cause that's what I just figured out.
To evolve Eevee into a Leafeon, you must level it up in Eterna Forest (Diamond, Pearl, Platinum) or in Pinwheel Forest (Black, White, Black 2, White 2). These forests contain a landmark known as the Mossy Rock which causes the transformation.

Even with a maxed out friendship/happiness, Eevee will still evolve into Leafeon if leveled in either of these forests, instead of an Umbreon or Espeon.

In Eterna Forest Eevee will not evolve into Leafeon if leveled up in the Old Chateau and in the path around the forest.
Eevee evolves into Leafeon once it's been leveled up by the Moss Rock. In Pokémon Diamond, Pearl and Platinum, the Moss Rock can be found in the Forest, in Pokémon Black, White, Black 2 and White 2, it can be found in Pinwheel Forest and in Pokémon X and Y, you can find it in Route 20 which is also classified as the Winding Woods.