This page of the Assassin's Creed Valhalla guide provides a detailed walkthrough of The Baker's Plaint quest.
Reward for completing the quest: 3400 XP.
Tarben, a baker from Ravensthorpe, asks for help with a few simple things. This quest can be accessed only after building a bakery in the settlement.
Follow the man to the port and take the boat down the river to meet the grain merchant. You can listen to Tarben's conversation with Eivor on your way there.
Once Tarben strikes the bargain, get back on the boat and sail to the Ravensthorpe settlement. The mission will end after arriving at the shore and having a short conversation with the baker.