you can't. the only way is to trade with emerald.
Gloom into Bellossom & Sunkern into Sunflora
You should evolve sunkern when you are in need of a strong grass type, and when you have a spare sun stone on hand. In Pokemon Ruby, Sapphire, and Emerald, you can get a sun stone in the Rocket Institution in Mossdeep City.
you trade sunkern from Pokemon gold/silver/crystal and evolve it with a sun stone
Well there is only about 3 Pokemon that evolve with the sun stone and 2 in Pokemon sapphire gloom and sunkern which turn into bellossom and sunflora.
A sunstone evolves an Eevee into an Espion in Pokemon gold and silver. this will not work in firered/leafgreen. It could also be done in ruby/sapphire/emerald though I'm not sure.
SUNKErn SUNKErn SUNKErn
Do whatever you want.
Sunkern is a Grass type pokemon.
Sunkern, the Seed Pokemon evolves into Sunflora, the Sun Pokemon. Both Pokemon are plant type pokemon.
Sunkern evolves via the sun stone.
Sunkern evolves into a sunflora when a sunstone is used on it.