Ok follow my step go 119 in the river you got two choices dead tape battery or the harder newly bought one (pokemon ame tape have battery)
stand 1 block from any rock face it you can use any rod but i prefer old rod
fish at least 4 time every rock
difference between dead battery and newly bought
|||||||||Dead battery- you will know because when you turn on your pkmn you can see a warning thsi is good because feebas won't move but if you pkmn emerald is newly bought then you should hurry up they probably transfer every day
trust me playing pkmn for 3 years
You can catch Feebas in Route 119. It can be caught by a fishing rod, but it has a rare availability.