There are 2 ways to evolve Feebas. The Beauty Condition method, and the Prism Scale method.
The Beauty Condition method is related to Contests. You use Berries to make treats for the Pokemon called Poffins. Blue ones raise the Beauty stat used in Contests. When you raise the Beauty of a Feebas high enough, then have it gain a level, it will evolve into Milotic. However, this method is only available in games that contain the Contests and their associated mechanics, and as such, is not available in Sun and Moon.
The Prism Scale method is much simpler. Obtain the Prism Scale item, give it to Feebas to hold, then trade it. Just like any other Trade-Evolution. Because Sun and Moon do not use Contests, this is the only way to evolve a Feebas to Milotic.