The mission in Chapter 2 will act as a tutorial mission to fishing in Red Dead Redemption 2. You will learn all sorts of things about fishing. Once the mission is done fishing will be unlocked in RDR2.
You can do fishing after that at any time. Simply go to a lake or a river body and take out your fishing pole by opening the weapon’s wheel and going into the items section and taking it out.
After you take out your fishing pole equip a bait by holding L1 / LB and select any bait.
The process of fishing in Red Dead Redemption 2 PC isn’t complicated just press L2 / LT and aim at a point you want to fish and cast using R2.
After a while, you will feel a bite and you will have to press R2 / RT so that you can hook the fish in. The main thing to do here is to tire out the fish so that you can reel it in.