George's guide is decent for finding the fishing pole, but doesn't really explain how to fish. I put together this alternate video that also tries to capture HOW to catch a fish in Dreamlight Valley.
To catch a fish, approach a pond and press
to cast your line, preferably towards a bubbling circle in a pond to guarantee you hit a fish. This will start a small circle moving away from you indicating where your lure will land. Once that small circle hits the larger circle in the pond, release
to cast your line. There is a small period of waiting until a fish bites. When it does, press
to start the actual mini-game. A large thin circle will appear and start shrinking towards a smaller, thicker circle. When the two overlap, the thicker circle will turn green. When it turns green, press
again. This may repeat several times. If you keep timing it right, you will be rewarded with a caught fish.
Simply repeat this 1800 times (or more if you catch seaweed or other items occasionally) and you should snag this very grindy achievement.