How do you catch a feebas in Pokemon LeafGreen?

You can't catch Feebas in Pokemon LeafGreen. Feebas was introduced in Generation III games (Ruby, Sapphire, and Emerald), and Pokemon LeafGreen is a Generation II game.

If you're looking to catch Feebas, you'll need to play one of the Generation III games or a later game that includes Feebas.