Can be Obtained From: Edit
In order to use this Tool, first equip it, then find a spot marked "Fish - Fishing Net", usually represented visually by some Starfish in the Water, and Left Click it to initiate the "Catch" action, you will continue automatically Fishing until you either disrupt your actions (eg. Walking Away), or run out of Inventory Space (Pet Inventory doesn't auto-fill).
To locate the correct Fishing spot, you might need some assistance more than just keeping your eyes open for Starfish, fortunately if you have loaded your Mods, you can use "/find Fish - Fishing Net" or "/find Fishing Net" to locate a valid location. If it doesn't offer you an exact Position at first, then try heading for one of the Maps that it lists and use that Command again when you get there.