Purchase the upgraded fishing rod and reel (costs 20,000 Gil) from the Vesperpool Lake fishing shop to make catching larger fish easier. It is also recommended to purchase the "Stinker: Great Maiboro" lure from the same shop, as it works really well at Vesperpool Lake. Equip them and start fishing at Vesperpool Lake, East Bank. This is in the north-western corner of the Cleigne map. Vesperpool Lake is the best fishing spot in the game. The more rare and large fish you catch, the greater skill increase you will get. Take a break at a campsite to increase your rank to make catching easier, then continue catching fish. Keep doing this for approximately 3 to 5 hours (depending on your luck) to max out the fishing skill.