In order to get feebas to evolve, u have to trade it with someone while it is holding the prism scale.
Using poffins/pokeblocks, max out Feebas's beauty condition and level it up after it hits max
In order to evolve feebass first max out its beauty then level it up it should evolve.
You Don't
the best way is to fish in every square (1 step = a square) in the lake next to the weather institute ( were you get that weather Pokemon after saving the scientist from team aqua.) then to evolve it max out its beauty and level it up during the day.
U feed pokeblocks that max out beauty like blue pokeblocks
get feebas then max out its beauty.
There is no beauty ranking in Pokemon Soul Silver therefore there is no action replay code for max beauty. Pokemon Soul Silver was released in 2010.
http://www.neoseeker.com/Games/action_replay/
blue pokeblocks to max beauty
To evolve feebas, you have to max out it's beauty, by feeding it a lot of pokeblocks that promote beauty.
Get a Pokemon to max out beauty (preferably feebas since it can evole when getting it and training at least one level) by feeding it pokeblocks. Pokeblocks can be made in lilycove. Go to Slateport and talk to the Chairman (if i remember right) of the Pokemon Fan Club with the Pokemon that has max beauty in front. He will give you a blue scarf after the conversation.
sure u can capture a feebas and evolve with max beauty (poffin) train the feebas (with beauty already in max) one level and it gotta evolve