Instructions
Beauty Statistic
Catch a Feebas of a modest, mild, rash or quiet nature. You can find out the nature of your Feebas by viewing it in the Pokedex screen.
Go to a contest hall found in the towns across the region. Walk up to the berry mixing platforms and press the "A" button to initiate the berry mixing process.
Mix the berries in your bag to make Pokeblock or Poffin to feed to your Feebas. For the best results use Ganlon berries and Blue or Indigo Pokeblocks to gain the biggest boost to your Feebas' beauty stat.
Continue the process of mixing the berries and feeding them to your Pokemon until you max out the beauty stat. You will be able to tell when you have achieved this because you will no longer see the message telling you it has improved after feeding your Feebas.
Level up your Feebas to the next level either through battle or by using a Rare Candy. Once your Feebas reaches the next level it will evolve and transform into Milotic.
Trade
Go to the "Items" section of your bag and find the "Clean Scale." Press the "A" button and choose the option "Give this to a Pokemon."
Select Feebas from the party menu by highlighting it and pressing the "A" button to give Feebas the "Clean Scale" item to hold.
Find a friend who is willing to trade your Feebas for another Pokemon. Go to the exchange section of the PokeCenter and follow the on-screen instructions to complete the trade procedure.
Allow Feebas to evolve and transform into Milotic under your friend's care.
Trade Milotic back from your friend to acquire Feebas' transformation.