With a leaf stone
How To Make An Eevee Evolve To Leafeon In Pokemon Indigo?
A leaf stone doesn't work
You can't make an eevee evolve into a leafeon in that version. You can only do that in moss rock before you make your eevee keep being happy. Only do that in pokemon diamond or pearl versions
You can't you can get all the pokemon that evolve from eevee except leafion sorry
You need to go to mossdeep city go to one of the big rock and make your eevee lv 100 and then go to evolve it will evolve into leafeon. Its true I tried it. ^_^