Go to the Job Center to take on the job.
Once you've taken on the job, you'll need two items:
Once you have your gear, head to a beach and open your inventory Y. Take a Fishing Net using the “Take” interaction.
Open your interaction wheel and “Throw” towards the water to send your buoys into the water.
Throw all your buoys spaced apart. Once all your buoys are in the water, approach your buoys, and interact with “Deploy Net”.
Wait between 3 and 5 minutes, and interact with “Take Net” on the buoy. If the net isn't full, wait until it's completely filled.
Your empty buckets will automatically fill when retrieving full Nets.
Once your buckets are filled, you have several options:
The fisherman's job isn't limited to catching fish. It's also possible to fish for mullets, if you have a boat, by going to a mullet area, and following this same tutorial.
These mullets can then be tagged for sale. To do this, buy a fish tag at the "Fishing Store", place your Mullet on the ground, and interact with “Tag meat”.
In addition to mullets and fish, it's possible to fish for sharks, intended for sale. To fish for sharks, the principle is similar to that of mullets. However, you'll need untagged mullets as bait. Head to a Shark fishing area, deploy your net, and set up bait.
Upon retrieving your fishing net, if you're lucky, your net will catch a shark. If, however, you're not very lucky, the caught shark will escape. Be careful, shark fishing is regulated by the Sheriff's Department.
Following the same principle as for mullets, you can tag your sharks with a shark tag, and then sell them.
It's also possible to follow the same process in a turtle fishing area, but beware, turtle fishing is strictly prohibited by law!