The thing that I like regarding these Feebas shenanigans is that once you find the spot, you can literally farm Feebas and trade them away for much more valuable Pokemon/Items, due to a 50% chance of finding Feebas on the right spot. The downside though, is that it's very easy to miss a spot because of the 50% encounter rate. I remember going through Route 119 twice, and found that I missed the spot on the second time because even after fishing five times on that spot, no Feebas.
As cliche and probably unhelpful this answer may be, it's that you need a whole lot of patience and time. I'd also suggest fishing minimum 7 times on each spot, if you really want to be sure and doubt RNG. It shouldn't take you longer than a few hours at best with decent enough luck, at least for me. Good luck, though, and it's definitely possible! ^~^