having finished Chapter 2
You must first get Special Lures from the Bait Shop in the Bayou, in the east of the map (location image below or see How to Get Fishing Bait & Lures).
Buy these 3 lures:
Special Lake Lure – $20.00 Special River Lure – $20.00 Special Swamp Lure – $20.00 The Special Lures are needed to hook legendary fish! It won’t work with your default lures. Bait is not needed, just the special lures. Legendary Fish aren’t attracted to bait.
Now head to the fishing spots below and equip the fishing rod from the item menu ( :l1: / :lb: to open weapon wheel, :r1: / :rb: to switch to item wheel, select fishing rod in bottom right) and then again press :l1: / :rb: to select the appropriate lure for the water type, at the bottom of wheel. Don’t select any bait. The correct lure will be pointed out in the location guide. However, any of the 3 special lures will work for every fish, using the correct lure just helps get a bite more quickly, you can try around with these if you wish.
https://www.youtube.com/watch?v=kGKKHzcPdRM&feature=emb_logo
Red Dead Redemption 2 has 26 Stranger Locations. This list includes a map that will show you where to find all 26 Strangers and the starting locations of their missions in RDR2.