Firstly fly to Dewford to start your quest.Go to the trendy guy and change the saying to something of your own(if it's not a saying you got through with you won't find it.)My saying is "Boring Luvdisk".
For Feebas you will need Either an Old rod or a Good rod(Old rod prefered).A hint to catching pokemon on the rod is by pressing A as soon as you get a bite.Net,Nest and Ultra balls can get a one catch first thing but for weakened ones use a Great or a normal Poke ball.If you buy 10 pokeballs all in one buy you get a Premeier Ball.Use a Magikarp at level 30-40 to weaken Feebas.Feebas is literally Magikarps duo,except it learns TM's and HM's and almost impossible to find.It is the strongest old roded pokemon since you find them at level 20-25.Old rod is found in Dewford and Good rod is found near the east of Mauville.The route Feebas appeares on is route119.
Here comes the long wait that will Frustrate you.There are 6 tiles on the entire game that contain Feebas,and they will change depending on the Dewford trend.However only one tile contains Feebas and it doesent change ever does depend on internal game number!The way to find it easily is to fish in one spot three times.If you ever come across it,Tell the world you've seen it!Fish there for more Feebas then catch 'em while they last!
1:Never give up
2:Try to resist the temptation to throw your Game boy out the Window.
Thais the Pokemon master