So first, you need to get into hardmode! Then you need to kill the 3 mechanical bosses, Plantera and the Golem. Then you need get an bug net from the Merchant for 25 silver coins and then you need to go to an undergound Glowing Mushroom biome and then you need to wait to see an a tiny Truffle worm and you need to get it with the bug net! Once you’ve done that, you need to go to the Ocean and then you need an Fishing pole! After that get the Truffle worm in your coins ammo inventory and then go fishing with the pole and wait for 3 seconds to get spawned Duke Fishron!
