ok so the easiest way i found to do this is by unlocking fishing in chapter 2 you’ll receive a mission called “A Fisher of Men" , after you have the fishing ability follow the directions below i couldn't figure out how to post photos but if you need photo help the link is included be sure to have atleast $50 to buy the special lures
First you are going to need to head over to gill landing on the shore of Flat Iron Lake in the Heartlands territory and talk to a fisherman named Gill fishing off the end of a dock
Once you talk to him, he will take you inside and tell you about him needing your help catching fish to stuff also at this time he will hand you a map of legendary fish locations to help you on your journey.
hop on your horse and head towards Lagras (location photo shown in link ) at this location you will see a bait shack where you can buy special lures for large type fish.
Then just cast the swamp special lure in and you should catch a big fish in a couple tries ! https://dontragegaming.wixsite.com/games/blog/rdr2fishing
here is a video walkthrough provided by maka if thats more your speed :)