Things You'll Need
Instructions
Go to Olivine City, which is south on Route 39. Enter the house north of the Pokemon Center, and speak with the owner to get the Good Rod. Alternatively, travel down Route 12. In the middle of the route you'll find a house. Inside is a fisherman who will give you the superior Super Rod. Both catch Corsola perfectly well.
Go to a map with Corsola in it. These maps include Routes 34, 40 and 19. Cherrygrove, Olivine and Cianwood City also have native populations of Corsola.
Wait until morning or night. The Corsola are only active during these hours, and if you go fishing during the day, you'll only catch other types of Pokemon.
Stand on a fishing platform, and use the Good or Super Rod. You'll reel in a random Pokemon, initiating a fight. This might not be the Corsola, and if it's not, you'll need to cast your line again until you find your quarry.
Attack the Corsola until it's almost dead.
Throw a Pokeball at the Corsola once it is weak to catch it. If the Corsola evades the ball, throw more until it's yours.