You can't, because Feebas is Pokémon that onlyappears when you fish for it while Surfing.
Go to route 119 in RSE and just use your super rod in every square in the water. In DPP just go the Mt Cornet 1st basement and again use your super rod in every square of water.
The other way you can get it is trade with someone who already has one.
Or you can get an action replay and cheat to get it.
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Go to Mt. Corpnet and go fishing. But, they are only found in 4 hidden spots. Fish 5 times in one spot. If a Feebas doesn't show up, Do this until you successfully find a Feebas. If you want more, keep fishing in that spot. The hidden spots change daily.
Mt.Cornet (There are 4 fishing spots.)
feebas is on the of the lake inside mt. coronet . you have to defog first. start on the first line starting from top to bottom . You Must have a fishing rod .
on route 119 there is a fisherman when you battle him he will have the feebas you can catch them right in front of where he is fishing unless you changed the catchy phrase in dewford town then its position will change
You can capture Feebas on Route 119 in Pokemon Ruby, this is dependant on the phrase in Dewford (that changes it's location) and is luck to find the correct tile that Feebas appears on when fishing.