How to Catch Milotic in Pokémon Ruby, Sapphire or Emerald

Go near Fortree city and use Surf in the water.

Use the Super Rod (or any rod).

Now try your luck in the tiles, go fish about 10 times in one tile. Keep doing this with other tiles until you find a Feebas.

Catch the Feebas.

Feed it Pokéblocks, and max its beauty.

If it doesn't like the Pokéblocks you feed it, then go to the same tile and fish till you get another one.

Remember that Feebas only likes Pokéblocks that are in shades of blue. This includes indigo, purple, or other colors related to that.

You can make Pokéblocks using these berries: Pomegranate and Corn.

Make sure you get more beauty stats from using right berry since any Pokémon can eat 12 Pokeblocks at max.

Start battling until it levels up. It should evolve into a Milotic.