The Beauty stat must be on max.
Then you level it up once.
Try making Pokeblocks using berries with high Dry flavour, like Wiki Berry or Cornn Berry.
If you can manage to get a Pamtre Berry, that would be super.
Also, make sure the Feebas's nature doesn't dislike Dry food.
These are the best natures for a Feebas to evolve:
Modest
Mild
Quiet
Rash
If you want your Milotic to be useful in battle, I recommend Modest nature (It lowers Attack, but raises Special Attack)