Well this question, just keeps on repeating
first go to mt.coronet(celestic side)and go to the underground pond, use surf on the first square and fish there for about 4-5 times. .if you still hasn't caught any feebas go to the next square and fish there, if still no feebas. .just keep doing this until you get it, it sorta like creating or drawing a imaginary grid lines. . .it's simple, but very challenging(i got a feebas via these method, and it's now a milotic)
hope this helps