it's actually quite simple, but very challenging!!
first go to the mt. where you can catch a feebas use surf, now in this pond you can only catch feebas in 4 random square, now begin at the bottom use your rod fish there for about 4-5 times if you still can't catch a feebas go to the next square and fish there, do this for every square,it's kinda creating a imaginary grid lines
this is how i got my feebas and it's now a milotic
(sorry if you don't understand it, it's kinda hard to explain)