The Farm Fortress Quest, is a quest where Granny Fields asks you to find and help her three grandsons. They're referred as Blue Farmer, Red Farmer, and Yellow Farmer. Its reward is a Hooked Scythe. To begin the quest, the player must head to Farm Fortress. They must find the Granny Fields, the quest giver that begins the Farm Fortress Quest. It's required for the player to have a fishing rod and a weapon to complete these objectives to finish the Farm Fortress quest.
The red farmer is located inside a building where Awoken Croc Carl may be found. This building is furthest from the market and the other buildings. The red farmer will ask the player to bring an apple to him. The apple can be obtained from Apple Bat or the Interdimensional Traveler. Once obtained, the red farmer will reward the player 3,000 gold.
The blue farmer is located inside the main building with the tunnel. The blue farmer will ask the player to bring a bass to him. The bass can be obtained by using a fishing rod. It's recommended to catch the bass at Topple Lake or Rubble Spring. Once obtained, the blue farmer will reward the player 3,000 gold.
The yellow farmer is located outside of a building, unlike the other two farmers. The yellow farmer is located on the side of a building near Greenhorn Grove. The yellow farmer will ask the player to bring him 10 Grey Rabbit Pelts. These rabbits are notorious for their spawn rate as they only spawn during the nighttime. If there are no Grey Rabbits to be seen, check Great Pine Forest. Once obtained, the yellow farmer will reward the player 3,000 gold and tells the player that he doesn't need the pelts and lets the player keep the pelts.
After completing these objectives, the player must interact with Granny Fields again. The player will tell Granny Fields that they had helped her grandsons and Granny Fields will reward the player a Hook Scythe. The quest ends there after completing the quest.
If words aren't enough, here's a link to a YouTube video of a player doing the Farm Fortress Quest. Please be aware that this video may be outdated and it would be best to rely on the paragraphs above.
Watch this video