Here's some directions:
Enter from Route 3. Go left. Go past the twins. You should see a noticeably different rock covered in moss. Level up your Eevee anywhere near that rock and it will evolve into Leafeon.
Go through the place where the twin is and go until you see a guy in red next to him or near him is a rock.Level up your eevee near the rock and you will get a leafeon.