To evolve Feebas beat the Elite 4. Then go to Kanto. In Kanto go to Pallet Town and go to the house next to Red's house. In there is Daisy Oak. She'll massage your Pokemon. You will probably have to do this a few times to make it evolve.
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Feebas does not evolve by level in Pokemon HeartGold. Feebas evolves when it levels up while it has a maximized beauty condition.
Give it a LOT o' haircuts then level it up.
In Pokemon Black And White, Feebas will evolve into Milotic by happiness, and, as i recall, berries, this process is easier if it were 'Poke Transfered' from Platinum, Diamond, Pearl, Soul Silver or Heartgold, as you can feed a Feebas Poffins (Beaty) until it won't take anymore. Than you level it up once and it will evolve into Milotic.
Feebas will only evolve with high beauty.
the only way to evolve it is to have the feebas at a high BEAUTY status for Pokemon contests. and the best way to do this is to have a feebas with a MODEST nature.